This 12 month calendar uses 4-5 week months, with the exception of December (in which 1 or two days will be added, depending on whether or not it's a leap year), but keeps the 365 days per year and 366 days on leap years.
The 4-week (28 day) months will be the months which are not divisible by 3:
The 5-week (35 day) months will be the months which are divisibe by 3, except December will have 1 more day (2 more days in leap years) than 5 weeks:
December ( plus 1 or two more days)
The nations adapting this calendar are recommended to declare the last weeks of March, June, Sept, and December as holiday weeks, using holiday names which approximately correspond to existing holidays, such as Easter, Christmas / New Year holidays. The December holiday week will have an addition day (or 2 days in leap years).
Why use this calendar?
Four-week months: 28 * 8 = 224
Five-week months: 35 * 4 = 140
224 + 140 + 1 = 365
11 months of the year will be divisible by 7, so people using it could easily calculate the day of the week any certain date, as long as they know what day of the week it was on January 1st. This method would even work for the extra day or two in December. However, each year will begin on a different weekday, but this is much better than every month starting on a different weekday.
A 13 month year with 28 day months (except for December) was considered but since 13 is a prime number, it would not be practical as it would not be divisible neatly into halves, quarters, and thirds into whole months. The last week of the 35, 36, or 37 day months would be deemed holiday weeks to deal with the problem of receiving the same monthly salary, while working for 7, 8, or 9 more days than the other 28 day months.