Calendar Calculator

What this calendar page draws

Most date tools answer one question. This one draws the month: a seven column grid, one cell per day, blanks where the previous and next month would sit, and an ISO week number beside each row when you switch the start day to Monday. You pick the month and the year, the grid is rebuilt, and the panel above it tells you the four numbers that define that month.

That makes it the page to open when you want to see a month rather than measure one. If you need a single figure, the site already has narrower tools for it. The weekday calculator gives the day of the week for one date and shows the arithmetic behind it. Week number 2026 covers ISO weeks for this year only. Days left in the year counts down to December 31. And calendar countdown measures the gap between two dates in days, weeks and business days.

This page does something those four cannot: it shows how the days sit on a wall calendar, including the empty cells at the start and the end, the weeks that straddle two months, and the holidays that fall inside the month you are looking at.

The four numbers that define a month

Every month grid is built from the same handful of inputs, and you can work all of them out from the year and the month number alone.

Days in the month. January 31, February 28 or 29, March 31, April 30, May 31, June 30, July 31, August 31, September 30, October 31, November 30, December 31. There is no memory trick that beats that list, because the pattern is historical rather than mathematical.

The weekday of the 1st. That sets how many blank cells come before day 1. October 1, 2026 is a Thursday.

The offset. Count the empty cells in the first row. On a Sunday-start grid, Thursday is four cells along, so October 2026 opens with four blanks. On a Monday-start grid the same month opens with three.

The row count. Add the offset to the number of days, divide by seven, and round up. For October 2026 on a Sunday-start grid: (4 + 31) / 7 = 35 / 7 = 5 rows. The grid needs 35 cells and the month fills 31 of them, so four cells stay empty at the end.

Weekday totals follow from the same data. A 31 day month always holds either 21 or 22 weekdays, never more and never fewer, because 31 days is four full weeks plus three extra days.

Sunday start against Monday start

US wall calendars start the week on Sunday, which puts Saturday and Sunday at opposite ends of the row. ISO 8601, the international standard behind week numbers, starts the week on Monday. The same month therefore looks different depending on which convention you choose, and only the Monday layout lines up with week numbers.

Switching the start day moves every date one column to the right or left and changes the offset. It does not change the number of days, the weekday of the 1st, or the weekday count. It can change the row count, because a different offset means a different number of trailing blanks.

February 2026 and February 2027 make the point best. Both are 28 day months, both fit in exactly four rows if you pick the right start day, and each needs five rows on the other convention. A four row February happens when February 1 lands on the first day of your week.

Where the leap day lands

February carries the extra day because the old Roman calendar put it there, and the Gregorian rule has kept it since 1582. A year that divides evenly by 4 is a leap year, except century years, which are leap only when they divide evenly by 400. The US Naval Observatory states the rule and the examples: 1700, 1800, 1900 and 2100 are common years, while 1600, 2000 and 2400 are leap years.

So 2026 is a common year with 365 days. 2028 is a leap year with 366, and February 2028 has 29 days. The next century problem is 2100, which nobody reading this will need, and the last one was 1900.

Because a common year is 365 days, which is 52 weeks and one day, a fixed date shifts forward by one weekday each year. October 21, 2026 is a Wednesday, so October 21, 2027 is a Thursday. Cross a leap day and the shift is two days instead: October 21, 2028 is a Saturday, because February 29, 2028 sits between the two dates.

Four, five or six rows

The row count is the part people notice, since a six row month makes the grid taller and changes how a printed page is laid out. The formula is the same every time: rows = (offset + days) / 7, rounded up to a whole number.

Four rows only happens in February, and only when February 1 falls on the first day of the week and the month has 28 days. That is February 2027 on a Monday-start grid: (0 + 28) / 7 = 4 rows, with the last cell of the last row landing exactly on the 28th.

Six rows happens when the month starts late in the week. May 2027 starts on a Saturday, so on a Sunday-start grid it opens with six blanks: (6 + 31) / 7 = 37 / 7 = 5.29, and rounding up gives 6 rows. November 2026 is the same story, with six blanks before the 1st and 30 days after it: (6 + 30) / 7 = 36 / 7 = 5.14, which also rounds to 6.

A six row grid holds 42 cells, so a 30 day month leaves 12 of them empty. That is why printed calendar pages sometimes look half full at the bottom and sometimes run right to the edge.

Federal holidays inside the month

The grid marks federal holidays for 2026 and 2027, using the observed dates from the Office of Personnel Management. There are 11 of them, set by federal law in 5 U.S.C. 6103, and most of them land on a Monday or a Thursday.

When a holiday falls on a Saturday, federal employees usually take the Friday before it. When it falls on a Sunday, they take the Monday after. That is why Independence Day shows up on Friday, July 3 in 2026: July 4 is a Saturday that year. The same rule moves Christmas to Friday, December 24 in 2027 and Juneteenth to Friday, June 18.

October 2026 is a good month to check the arithmetic against. It has 22 weekdays and one federal holiday, Columbus Day on the 12th, which leaves 21 working days for anyone on a Monday to Friday schedule. For a full business day count between two dates, the business days calculator handles the subtraction.

Week numbers on the side of the grid

Switch the start day to Monday and a week number column appears on the left. Those are ISO 8601 week numbers, defined by the ISO standard: weeks run Monday to Sunday, and week 1 is the week that contains the first Thursday in January.

The rule has one odd consequence worth knowing. A year can have 52 or 53 weeks, and the days around New Year can belong to the previous year's numbering. January 1, 2027 is a Friday, so it sits in the week that began on Monday, December 28, 2026, which is week 53 of 2026. A calendar built for 2027 will therefore show a first row numbered 53.

2026 is one of the 53 week years, because January 1, 2026 is a Thursday. Years like that come in clusters, and the next one is 2032. If your payroll, sprint board or delivery schedule runs on week numbers, this is the reason a five column report sometimes contains week 53 and sometimes skips it.

The week number column carries the number of the week that the row starts in, even when that week began in the previous month. That keeps adjacent months consistent: the last row of September 2026 and the first row of October 2026 show the same number for the days they share.

Printing the grid and planning with it

The grid is plain HTML, so browser print produces a clean page: no images, no fonts to load, and the weekday headings repeat on the printed sheet. Set the start day first, because switching it after printing changes every column. If you need the week numbers, print the Monday layout, since a Sunday-start grid drops that column by design.

Three planning jobs come up again and again. Pay periods: a biweekly schedule moves through a month in two or three chunks, and the grid shows at a glance which Fridays are paydays. Rent and bills: the due date is fixed, but the weekday decides whether a payment lands before a weekend. School terms and shift rotations: a six week block is 42 days, which crosses two month grids and sometimes a holiday.

One habit saves mistakes. Count from the date, not from the row. A date in the last row of a six row month can sit in the following month's first week, and a date in the first row can belong to the previous month's final week. The week number column tells you which side of the line you are on.

For a date that has already passed, or one far in the future, the same arithmetic holds. The grid is generated from the calendar rules, not from a table of dates, so 1900 and 2200 both work, and only the holiday marks are limited to the two years loaded here.