For Regulation E change in terms notices (1005.8) is the 21 "days" - calendar days or business days? Also, is this from the mailing date of the notice or receipt by the customers?

You must mail it at least 21 calendar days before the change.

Agree with Adam. If the term isn't specific or defined to mean business days, and this change in terms is not, it's calendar days.

Reg E pre-dates Reg DD which uses a 30-day change period. If making Reg E and Reg DD changes, you can break the two up, or use 30-days in place of 21 and give the consumer more time.