About

Generating a correct calendar grid requires resolving the weekday of the first day of any month across the full Gregorian range. An off-by-one error in the day-of-week computation shifts every date in the grid, producing a calendar that looks plausible but is factually wrong. This tool computes the weekday offset using modular arithmetic consistent with the Gregorian reform rules: a year y is a leap year if divisible by 4, except centuries not divisible by 400. Events are keyed to ISO 8601 date strings (YYYY−MM−DD) to avoid locale ambiguity. The output is a standards-compliant HTML table suitable for printing at A4 or Letter size.

Limitation: this tool covers the proleptic Gregorian calendar only. It does not account for the Julian-to-Gregorian transition gap (October 1582 in Catholic countries, later elsewhere). For dates before roughly 1582, the weekday assignment may differ from historical records. Pro tip: if you plan to print calendars for wall use, set your browser print margins to “None” for maximum grid size.

Formulas

The number of days in February depends on whether year y is a leap year. The leap year predicate L(y) is defined as:

L(y) = {

TRUE if y mod 400 = 0FALSE if y mod 100 = 0TRUE if y mod 4 = 0FALSE otherwise

The day-of-week for the first of any month is computed via the native Date constructor, which implements the proleptic Gregorian calendar. Given month index m (0 - 11) and year y, the weekday w of day 1 is:

w = Date(y, m, 1).getDay() where w ∈ {0, 1, …, 6}

The grid offset o (number of blank cells before day 1) when the week starts on day s (0 = Sunday, 1 = Monday) is:

o = (w − s + 7) mod 7

Where y = year, m = zero-indexed month, w = weekday index (Sunday = 0), s = configured week start day, o = leading empty cells in the grid.

Reference Data

Month	Days	Notes
January	31	New Year's Day (1st)
February	28 or 29	Leap day on 29th when y mod 4 = 0
March	31	Spring equinox ≈ 20th
April	30	Earth Day (22nd)
May	31	Memorial Day (last Mon, US)
June	30	Summer solstice ≈ 21st
July	31	Independence Day (4th, US)
August	31	No major fixed holidays
September	30	Labor Day (1st Mon, US)
October	31	Halloween (31st)
November	30	Thanksgiving (4th Thu, US)
December	31	Christmas Day (25th)
Leap Year Rules
Rule 1	y mod 4 = 0 → leap candidate
Rule 2	y mod 100 = 0 → not leap (exception below)
Rule 3	y mod 400 = 0 → leap year
Common Week Start Conventions
ISO 8601	Monday is day 1
US / Canada	Sunday is day 0
Middle East	Saturday start common
Notable Leap Years
2000	Leap (divisible by 400)
1900	Not leap (century, not ÷ 400)
2024	Leap
2100	Not leap

Frequently Asked Questions

Century years (1800, 1900, 2100) are NOT leap years unless they are also divisible by 400. The year 2000 was a leap year because 2000 mod 400 = 0. The year 1900 was not because 1900 mod 400 = 300. This tool applies all three Gregorian rules in the correct precedence order.

Technically yes, but the results represent the proleptic Gregorian calendar, which projects current rules backward. Historical calendars before the Gregorian reform (October 15, 1582) used the Julian system, which lacks the century-year exception. Weekdays shown for pre-1582 dates will not match historical records.

A month with 31 days whose first day falls on a Friday (Sunday-start) or Saturday (Monday-start) requires 6 rows to display all dates. February in a non-leap year starting on Sunday (Sunday-start calendar) fits exactly 4 rows. The grid dynamically expands to accommodate the actual layout.

Events are persisted to localStorage under a namespaced key. Clearing browser data or localStorage will erase them. Use the Export function to download your calendar as an HTML file before clearing data. There is no server-side backup.

No. Events are keyed to ISO 8601 date strings (YYYY-MM-DD). Changing the week start day only rearranges the visual grid. All events remain on their correct dates regardless of whether the week begins on Sunday, Monday, or Saturday.

Set orientation to Landscape for wider grids. Set margins to None or Minimum. Disable headers and footers in your browser print dialog. The tool includes @media print styles that automatically hide navigation buttons and optimize the grid for paper. A4 and US Letter sizes are both supported.