When a constituent creates a sustaining gift, changes in the length of the month can make it confusing to know when recurring payments are scheduled to be processed.
For all sustaining gifts, barring any card processing issues, the next attempted day is the same as the original gift day. So for gifts given on the 1st through the 28th, the next attempted day will always be the same day. If the gift day is the 31st and the current month has less than 31 days, the gift will process on the last day of the month. For the 29th, this is only true once a year, but for the 31st, the date will fluctuate often. Here is an example that has a gift day of the 31st:
Original gift January 31st
Second gift Febrary 28th/29th (depending on Leap Year)