Odds of any given month having an occurrence = 1/7
Odds of it occurring in a non leap year February = 3/48 = 1/16
1/16 * 1/7 = 1/112 = once every nine years and four months in
11/13 = 22/26 = 33/39
Three is the maximum and they have to fall in February, March and November as they do in 2009 (in leap years the max is still three, but the months are January, April, and July as in 1984 and 2012) . A normal year will have only one or two. The most is three. The least is one.
Tautology
1 in 36
0.149