For each day I calculated the size of that day's move in the S&P relative to the mean and SD of the previous 250 trading days. Here are the frequencies of moves that were -2, -1.5, +1.5 and +2 SDs outside their 250-day means.

I adjusted the frequencies to account for the differing numbers of days counted in total for each month. For example, there were 1180 January days, and the average for all months was 1155.083 days. So the January frequency was adjusted by the factor: 1155.083/1180 = .979.