13 months × 28 days = Every date falls on the same weekday each year
Each month has exactly 28 days (4 complete weeks). The 13th month "Sol" is inserted between June and July.
The last day of the year (after December 28) is "Year Day" - a worldwide holiday outside the weekly cycle.
In leap years, "Leap Day" occurs after June 28 (before Sol 1). It's also outside the weekly cycle.
Every year is identical! The 1st of each month is always Sunday. The 13th is always Friday. Scheduling becomes trivial.
Every month follows the same pattern. Day 1 is always Sunday:
| Sun | Mon | Tue | Wed | Thu | Fri | Sat |
|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| 8 | 9 | 10 | 11 | 12 | 13 | 14 |
| 15 | 16 | 17 | 18 | 19 | 20 | 21 |
| 22 | 23 | 24 | 25 | 26 | 27 | 28 |
The International Fixed Calendar was proposed by Moses B. Cotsworth in 1902 and later championed by George Eastman (founder of Kodak). Eastman used it at Kodak from 1928 to 1989. The League of Nations considered adopting it in the 1920s-30s but faced opposition from religious groups concerned about the "blank days" disrupting the weekly Sabbath cycle.