Does the Swiss Travel Pass FLEX remain valid if two months overlap while I am in Switzerland?

Yes. All Swiss Travel System tickets remain valid over the turn of the month. For example, a Swiss Travel Pass for 8 consecutive days is valid from 30 January to 6 February. The Swiss Travel Pass Flex can be validated on all available travel days within the period of a month (28 to 31 days). For example, a Swiss Travel Pass Flex can be validated on 8 days of your choice during the period of 25 April to 24 May.

Swiss Travel System

Swiss Travel Pass