The 54-Week Calendar contains exactly 54 weeks per year. Some weeks are shortened to 6 days, though. In a leap year with 366 days, all adjacent months contain exactly 9 weeks (or 61 days) together and every month has one week with only 6 days in it. In common years, the last week in the last month there also contains just 6 days.
54 × 7 – 13 = 365
The usual pattern of days per week is 6-7-7-7-6-7-7-7-7, i.e. the first week of odd-numbered months and the week shared with the following even-numbered month are short. Odd months have 30 days, even numbered (except the 12th) have 31 days.
Alternatively, months could be defined to have alternating length of 5 and 4 weeks exactly, where the former includes 2 short ones, i.e. months have 33 days or 28 days (and the last one just 27 days in common years).
It would also be possible to divide the year into 9 months of 6 weeks each, but five would have 40 days and four had 41 days. With the layout described above, the 1st, 4th, 7th and possibly 9th month would be shorter. The position of the short weeks would differ from month to month.