The 400 Day Calendar is a calendar that has exactly 400 days. Due to the fact that the 30-31 pattern is unable to make 400 days in a year, we have 9 months of either 39 or 40 days. Due to the fact that the 39-40 pattern is only able to make the calendar 395 or 396 days long, October has 44 days.
January: 39
February: 39
March: 40
April: 40
May: 39
June: 40
July: 40
August: 39
September: 40
October: 44
Month days
4 of the months have 39 days. This also means that only 156 out of 400 days are in 39 day months. 200 are in 40 day months, and 44 are in 44 day months (as there is only one 44 day month.) Since half of the days are in 40 day months, it is almost safe to say that every month is around 39.5 days long.
Month weeks
The closest multiple of 7 from 39.5 times 10 (395) is 392, so you might wanna say that a month is 5.6 weeks long. But, if you wanna assume all of the months have 40 days (since there are 400 days divided into 10 months), the closest multiple of 7 is 399, as 400 is only 1 number following 399, and you might round it to 5.7 weeks if you want to.
Which means that a full year is 57 weeks. Yes, 57 weeks. If you wanna go with the 39.5 day week calculation, you might say 56.428 weeks to be exact. Though, that would make this calendar only 395 days long.