How calendar grids work
A month grid looks fixed, but it’s simple arithmetic: blank cells, then days, then rows. Change the month and week start below and watch the grid rebuild.
1 August 2026 is a Saturday, so the grid starts with 6 blank cells. 6 + 31 days = 37 cells. Divide by 7 and round up: 6 rows. The pink days spill past cell 35, into a sixth row.
Leap years: the three-question rule
The Gregorian calendar adds 29 February to keep the calendar year in step with the solar year of about 365.2422 days. Every fourth year overshoots slightly, so century years skip it unless they divide by 400. Try 1900, 2000, 2028 and 2100.
- Divisible by 4? Yes
- Divisible by 100? Yes
- Divisible by 400? No
1900 is not a leap year: February has 28 days.
Week 1 and the Thursday rule
Planners that print week numbers usually follow ISO 8601: weeks run Monday to Sunday, and week 1 is the week containing the year’s first Thursday. Equivalently, it’s the week with 4 January in it. That’s why 1 January can belong to week 52 or 53 of the year before.
1 January 2027 is a Friday. Its week’s Thursday falls on Thu 31 Dec 2026, still in 2026. So 1 January is in week 53 of 2026, and week 1 of 2027 starts the following Monday.
Questions
What is the most rows a month can need?
Six. Any month fits in six rows of seven, because even a 31-day month starting on the last day of the week occupies 1 + 30 = 31 more cells, at most 6 × 7 = 42 in total.
Can a month fit in four rows?
Only a 28-day February that begins on the first day of your week, for example February 2026 with a Sunday start or February 2027 with a Monday start.
Does the week-start setting change the ISO week number?
No. ISO week numbers always count Monday-to-Sunday weeks. A Sunday-start calendar can still print them, but the number then belongs to the Monday after that Sunday.
Ready to print one? The monthly calendar maker handles all of this for you, including six-row months.