The International Fixed Calendar, a propoal that was floated at the League of Nations and even tried for internal business at Eastman/Kodak, is a calendar designed to simplify the Gregorian Calendar that is widely used in international affairs today by dividing the same calendar year of 365 or 366 days into thirteen months, each with a fixed length of 28 days.
This arrangment means each month is exactly four weeks long, and always begins on a Sunday and ends on a Saturday. This means every month fits the same calendar grid, and you can always tell what day of the week a date is without checking the calendar. The 3rd of the month is always a Tuesday, the 19th is always a Thursday, and so on. Every month on the International Fixed Calendar looks like this:
| Su | Mo | Tu | We | Th | Fr | Sa |
|---|---|---|---|---|---|---|
| 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 Calender months all have the same names as the Gregorian Calendar months, except that there's a new month called Sol between June and July. So the International Fixed Calendar months are: January, February, March, April, May, June, Sol, July, August, September, October, November, December.
So how is it possible to divide a year of 365 or 366 days into 13 equal months? Two important exceptions have to be made. First, every leap year (calculated the same way as Gregorian Calendar leap years) has an extra day called Leap Day, between 28 June and 1 Sol. Secondly, every year ends in Year Day, the day after 28 December. Leap Day and Year Day are both special in that neither of them is assigned a day of the week. Under an International Fixed Calendar system, both would likely be observed as civic holidays.
I like the International Fixed Calendar because it simplifies planning and makes it easier to quickly draw your own monthly calender relative to the Gregorian Calendar. But, because its days of the week are not synchronized to calenders in common usage today, adopting it would put the days of the week used to time weekly religious observances (such as the Jewish Shabbat) out-of-sync with the civic days of the week. To me this is the most serious drawback. Another issue with the International Fixed Calendar is that the year can not be divided into four financial quarters with the same integer number of months, but to me this seems an advantage, because financial quarters are a dreary concept anyway. The society in which I imagine using the International Fixed calendar is a society where necessary work is equitably distributed and people generally have abundant free time to meet and collaborate on creative projects, enjoy art together, and so on. And when we plan those meetings it's easier to do, because the months all have the same number of days and the 27th is always a Friday.