The 26-day month calendar is a calendar in which every month has 26 days without exception. Each day of the month is identified by a letter of the alphabet rather than a number. So the first day of a month is A the next day is B etc. and the last day is Z.
Most years have 14 months, but occassionally there is a leap year with 15 months. Each year begins at the same day of week up to the next leap year, then the years begin two days of the week earlier.
Each leap year occurs 21 years after the previous leap year, except every other leap year beginning Sunday, which occurs 20 years after the previous leap year. This creates a 293-year cycle with 14 leap years. The 14 leap months make up a 294th year of 364 days. The resulting mean year is 365 71/293 = 365.24232 days.
The 293-year cycle has only 41 years beginning with a Sunday compared with 42 years beginning with each other day of the week.
42 years including two leap years have 590 months and hence 59 Maya tzolkins.