What is your favorite time of day? "What" in a question asks for information about something that you have to specify . So, what is it ? Night time , day time or morning time ? Or, the time after your afternoon nap ? Its asking for a specific "thing"
When is your favorite time of day? "When" in a question ask for the time itself > 8 pm , 11 am or when I have taken my afternoon nap .
They are similar coz they are asking for the same answers ( what sort of time<> what time )