[Freak_NL]

Isn't this pretty much what these thermostats already allow? I have a new Honeywell Thermostat which basically does what the twenty years old one it replaced does with a few added conveniences in terms of UI. It has those wake/leave/return/sleep instants for each weekday (but also adds an optional second leave/return pair), and it has an option to override the day programme to 'holiday', which is essentially an eighth programmable weekday you can activate at any time.

Your use case is possible with that. Just set the standard program to 15°C, and activate the holiday set to whatever you need whenever you want. Configure it to go to 15°C at some sensible time in the evening, so it won't go on even if you forget it.

[meindnoch]

15 C is very cold. Are you American?

[Freak_NL]

15°C would be the standard setting of 'no heating' (just keep the house warm enough to avoid mould and such). The 'holiday' programme would be 21°C or whatever is desired. That would effectively turn the thermostat into one where turn on the heating by putting it in 'holiday' mode, and can turn it off by exiting that mode or just letting it revert to 15°C after 23:00 or so.