There are 42 possible numbers, every number between 2 and 43 inclusive.
Following is the list of the numbers that can be constructed each month.
January: 2 to 32
February: 3 to 30 (ignoring leap year)
March: 4 to 34
April: 5 to 34
May: 6 to 36
June: 7 to 36
July: 8 to 38
August: 9 to 39
September: 10 to 39
October: 11 to 41
November: 12 to 41
December: 13 to 43
The most frequent numbers are those from 13 to 30 - they will occur once every month of every year. The most infrequent numbers are 2, 42, and 43. Each occurs only once a year - 2 on January 1st, 42 on December 30th, and 43 on December 31st.