White and Rodgers programable thermostat
The specifics vary between brands &amp; models, but it is typical for a programmable thermostat to have 4 setpoints per day (time &amp; temperature) with the setpoints either for weekend (Saturday &amp; Sunday the same) and weekday (Monday through Friday) or each individual day of the week. Some thermostats give both options.
Since this happens specifically Monday 12:00am, it sounds like when the thermostat was programmed that the first Monday setpoint was missed. Note that many computerized systems (programmable thermostats being a simple example) actually start with Monday - see ISO week day for more details. This means that &quot;first setpoint on Monday&quot; may actually be &quot;very first setpoint in the thermostat&quot;.
In any case, the solution is to step through the programming routine for every setting of every day. In a worst case scenario (bug in the thermostat firmware that makes it always reset at 12:00 Monday with no option to change the temperature - extremely unlikely), set the desired temperature for 12:15am Monday (most thermostats I have seen use a 15-minute increment) and you will essentially solve the problem.