It just a spur of the moment thing.
Some guys do prefer a specific time. Some are too tired and late nights might not work. Some hate to get up or get it up in the morning, so early morning jaunts are taboo.
Being compatible is all about when the guy enjoys his playtime. If guys work different times of day, than that can put a crunch on the sex life. Getting things and urges in synch is the key to great sex.
So when is the optimal time for you? Does it really depend on the time of day or when you are the most horny or least tired? We guys have different body moods and some times may be better than others. I guess it all depends on the individual, the opportunity and the circumstances. Big hairy muscle hugs. Now that's the way I like it, unhuh, unhuh.