Under 2200.4(a)(1), when a period is stated in days, do I count the day that triggers the period as day 1?
No — you do not count the day that triggers the period; start counting the next day as day 1. See the rule in 2200.4(a)(1)(i). For practical use: begin the countdown on the calendar day after the triggering event and then count every day (including weekends and federal holidays) unless the rule says otherwise (2200.4(a)(1)).