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.