The regulation allows three options -- a calendar month (which I believe could also mean from the Nth of each month to the Nth of the following month), a statement cycle, or a "similar period of at least four weeks."
For savings accounts that don't have regular monthly statements, I recommend moving to a monthly period that can either use the calendar month or the Nth to Nth variation (with N corresponding to the expected savings statement cycle date).
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8