It's varied quite a bit over the last year. They always start on Monday, unless Monday is a holiday, in which case it will start on a Tuesday. Some months have had two classes a month, and some have had three. In all cases (except the first month of this hiring wave) the first class was on the first Monday of the month. The next class or two it has been pretty variable, with a bias towards the beginning of the month. To be more specific, from the start of this wave:

Jan '14: 2nd and 4th
Feb '14: 1st and 3rd
Mar '14: 1st, 3rd, and 5th
Apr '14: 1st, 2nd, and 3rd
May '14: 1st, 2nd, and 3rd
Jun '14: 1st and 3rd
Jul '14: 1st and 3rd
Aug '14: 1st, 2nd, and 3rd
Sep '14: 1st, 2nd, and 3rd
Oct '14: 1st and 3rd
Nov '14: 1st and 2nd
Dec '14: 1st and 2nd
Jan '15: 1st and 3rd