https://en.wikipedia.org/wiki/Internati ... d_Calendar
So, 13*28=364. The extra day, is New Year's Day (2 days if a leap year). New Year's Day and Leap Year Day do not count as days of the week, so they are not considered a Sunday or Monday or any other weekday. That way, the 1st of January (or the first day of any month) is always a Sunday and the last day of the month is always a Saturday. Likewise, the 2nd day of every month is a Monday, and so on.The International Fixed Calendar (also known as the Cotsworth plan, the Eastman plan, the 13 Month calendar or the Equal Month calendar) is a solar calendar proposal for calendar reform designed by Moses B. Cotsworth, who presented it in 1902. It divides the solar year into 13 months of 28 days each. It is therefore a perennial calendar, with every date fixed to the same weekday every year. Though it was never officially adopted in any country, entrepreneur George Eastman adopted it for use in his Eastman Kodak Company, where it was used from 1928 to 1989.
Unfortunately, 28 days is not exactly a lunar month, but it's kinda close. So the phases of the moon will tend to fall on close to the same day of the month, although shifting by about a day and a half per month.