Combining two consecutive months of 30 and 31 days, gives a 61-day double-month, bimonth or bimester. This can best be done with solilunar calendar designs that have 12 months with regularly alternating lengths:
For a leap-day calendar design, one of the six bimonths would need to omit a single day in common years: 6 × 61 − 1 = 365.
Since 61 doesn’t divide by 7 evenly, full-week double-months would need to be 56 or 63 days long, i.e. 2 × 8 and 4 × 9 weeks in a regular year of 52 weeks in total. They could be divided into single-months of 4 and 5 weeks each in any of a number of regular patterns.
Since 61 is a prime number, weeks of 5, 6 or 10 days each can be used alternatively, and the 61st day would be outside the week cycle. It would even be possible to use sub-groupings of 12, 15 or 20 days, but these are usually called neither week nor month, even fortnight would be a misnomer.
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