Month calendars with the week starting on Sunday

How it works

The formula

daysInMonth = 29 for February in a leap year, 28 otherwise
              30 for April, June, September, November
              31 for the rest

leap year   = year mod 4 = 0  and  (year mod 100 != 0  or  year mod 400 = 0)

firstColumn = 0 for a Sunday start, 1 for Monday, 6 for Saturday
              weekdays indexed Sunday = 0 … Saturday = 6

leadingBlanks  = (weekdayOfThe1st - firstColumn + 7) mod 7
cells          = leadingBlanks + daysInMonth
rows           = ceil(cells / 7)
trailingBlanks = rows x 7 - cells

for the dth day:
  offset = leadingBlanks + d - 1
  row    = floor(offset / 7) + 1
  column = (offset mod 7) + 1

ISO 8601 weeks run Monday to Sunday, and week 1 is the week holding the
year's first Thursday:

  isoWeekday = 1 for Monday … 7 for Sunday
  thursday   = dayNumber + (4 - isoWeekday)
  weekYear   = the calendar year that thursday falls in
  isoWeek    = floor((thursday - 1 January of weekYear) / 7) + 1

columns   = 8 when a week-number column is shown, otherwise 7
boxWidth  = (pageWidth  - 2 x margin) / columns
boxHeight = (pageHeight - 2 x margin - headingSpace) / rows

Every length is held as a whole number of micrometres up to that last
division. Every day number is integer arithmetic throughout.

What it assumes

• Dates are computed in the proleptic Gregorian calendar for every year from 1583 onwards, which is why that is the earliest year offered. Britain and its colonies kept the Julian calendar until September 1752, so for a month before then this tool disagrees with the calendar people were actually using at the time — by ten days in 1583 and by eleven by 1752.
• The day the week starts on is chosen explicitly rather than inferred from the language. CLDR does record a first day per territory, but it is keyed on region and not on language, so "French" alone does not answer the question — and the setting changes the row count, which makes guessing it wrong more than a cosmetic error.
• ISO week numbers always count Monday-to-Sunday weeks, whatever day the grid starts on. A Sunday-start calendar therefore shows, against its first row, a week number for a week that began the day before that row does.
• Every column is the same width and every row the same height, including the week-number column when it is switched on. Printed calendars often narrow the week-number column or the weekend, and none of those conventions is standard enough to compute from.
• Paper sizes are trimmed sizes. Your printer's own hardware border, usually 4 to 6 mm and often wider at the trailing edge, is not deducted, so a margin below about 5 mm will be clipped by the printer rather than by anything here.
• Month and weekday names come from the locale data built into the browser reading this page. A cut-down build of that data, which a few embedded and self-compiled runtimes ship, falls back to English without saying so.