A leap year starting on Monday is any year with 366 days (i.e. it includes 29 February) that begins on Monday, 1 January, and ends on Tuesday, 31 December. Its dominical letter hence is GF, such as the years 1940, 1968, 1996, 2024, 2052, 2080, and 2120 in the Gregorian calendar or, likewise, 2008 and 2036 in the obsolete Julian calendar.
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Gregorian Calendar
Leap years beginning on Monday, along with those that start on Saturday, occur least frequently - 13 out of 97 (≈ 13.4%) possible total leap years in a 400 year cycle. Their overall occurrence is thus 3.25%.
Julian Calendar
Like all leap year types, the one starting with 1 January on a Monday 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.