A leap year starting on Friday is any year with 366 days (i.e. it includes 29 February) that begins on Friday 1 January and ends on Saturday 31 December. Its dominical letters hence are CB. The most recent year of such kind was 2016 and the next one will be 2044 in the Gregorian calendar[1] or, likewise, 2000 and 2028 in the obsolete Julian calendar.
In this type of year, all dates (except 29 February) fall on their respective weekdays the maximal 58 times in the 400 year Gregorian calendar cycle. Leap years starting on Sunday share this characteristic.
Leap years that begin on Friday, along with those starting on Sunday, occur most frequently: 15 of the 97 (≈ 15.46%) total leap years in a 400-year cycle of the Gregorian calendar. Thus, their overall occurrence is 3.75% (15 out of 400).
For this kind of year, the ISO week 10 (which begins March 7) and all subsequent ISO weeks occur later than in all other leap years.
Like all leap year types, the one starting with 1 January on a Friday occurs exactly once in a 28-year cycle in the Julian calendar, i.e. in 3.57% of years. As the Julian calendar repeats after 28 years that means it will also repeat after 700 years, i.e. 25 cycles. The year's position in the cycle is given by the formula ((year 8) mod 28) 1).
Valentine's Day falls on a Sunday. This is the only type of leap year when Valentine’s Day falls in ISO week 6. They fall in ISO week 7 in all other leap years.
Christmas Day falls on a Sunday. This is the only type of leap year when Christmas Day falls in ISO week 51. They fall in ISO week 52 in all other leap years.
The First Sunday of Advent falls on its earliest possible date, November 27. This is the only leap year when The First Sunday of Advent falls in ISO week 47. They fall in ISO week 48 in all other leap years.
Gaudete Sunday falls on its earliest possible date, December 11. This is the only leap year when Gaduete Sunday falls in ISO week 49. They fall in ISO week 50 in all other leap years.
Rorate Sunday falls on its earliest possible date, December 18. This is the only leap year when Rorate Sunday falls in ISO week 50. They fall in ISO week 51 in all other leap years.
Mother's Day falls on its earliest possible date, May 8. This is the only leap year when Mother’s Day falls in ISO week 18. They fall in ISO week 19 in all other leap years.
Mother's Day falls on its earliest possible date, May 8. This is the only leap year when Mother’s Day falls in ISO week 18. They fall in ISO week 19 in all other leap years
President's Day falls on its earliest possible date, February 15. This is the only leap year when President’s Day falls in ISO week 7. They fall in ISO week 8 in all other leap years. This is also the only leap year when Labor Day that precedes this type of year to President’s Day in this type of year are only 23 weeks apart. They are 24 weeks apart in all other leap years.
Mother's Day falls on its earliest possible date, May 8. This is the only leap year when Mother’s Day falls in ISO week 18. They fall in ISO week 19 in all other leap years.
Juneteenth and Father's Day coincide on Sunday, June 19. This is the only leap year when Juneteenth falls in ISO week 24. They fall in ISO week 25 in all other leap years.
