1st monday to 2nd tuedays is 8 days except when the month starts with tuesday, then it is 1 day.
In the 28 years cycle when weekdays match the same dates compared to leap years (not counting the year%100 and year%400 issues) there are actually exactly 1/7th of the months that starts with tuesdays. So on average (6/7)*8 + (1/7)*1 = 7 days.
Why is it in the 28 years (336 months) cycle the 1st of the months falls on a monday exactly 48 times, tuesday 48 times, ..., and sunday 48 times, even though the length of the months switches in the weird pattern between 28,29,30 and 31 days long ?
Last fiddled with by ATH on 20190306 at 14:32
