MyStrengthBook also allows users to select days of week to train (e.g. Mon, Wed, Fri, Sat) if programing is for 4 days and you like consistency, but it also gives the athlete the option to plan on a continuous basis - i.e. only once one session is completed, can you log the second on whatever day you wish to choose. Sessions are still in order, but one week could be Mon, Tue, Wed, Thur and week two could be Tue, Thur, Fri, Sun ... completely depending on availability.

This is a fabulous idea, and it's something that almost convinced me to use BridgeAthletic over Trainheroic. They've got a version of this called Playlist Programs, where the order of sessions is prescribed, but the specific dates aren't. Clients select the session they want to do from the list of published sessions, which are listed and given numbers instead of dates.