A leap year starting on Wednesday is any year with 366 days (i.e. it includes 29 February) that begins on Wednesday, 1 January, and ends on Thursday, 31 December. Its dominical letter hence is ED, such as the years 1936, 1964, 1992, 2020, 2048, 2076, and 2116 in the Gregorian calendar or, likewise, 2004 and 2032 in the obsolete Julian calendar. This kind of year has 53 weeks in the ISO 8601 week - day format.
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Gregorian Calendar
Leap years beginning on Wednesday occur 14 times of the 97 (or ≈ 14.4%) possible total leap years in a 400 year cycle. Their overall occurrence is 3.5%.
Julian Calendar
Like all leap year types, the one starting with 1 January on a Wednesday occurs exactly once in a 28-year cycle in the Julian calendar, i.e. in 3.57% of years.
The final two digits of Julian years repeat after 700 years, i.e. 25 cycles.