I would recommend that you look at this previous answer I've done for it which also included one of our prime Ancient Warfare specialists.

Designated start time is utter bullshit, and I mean every syllable of that statement. This myth mostly came about as a result of someone pining for the good old days of civilized, romatic warfare that didn't exist - you can see similar thing happening with samurai these days.

That said, battles were fought at certain times because of practicality - it's really damn hard to coordinate units of men during the night. It has been done, but it wasn't easy, an army can well end up fighting itself, since thay can't see if the crowd in front of them are friends that got lost or enemy forces.

Winter campaigns were also not very common, simply because you're liable to loose a lot of your troops to weather before you even get to the enemy, sitting in his cozy castle, with all the harvest stored inside.

And there's one more thing, rarely talked about - field battles are rare. For them to happen, either both sides have to think they can easily win (otherwise one of them would withdraw to nearest fortification and we'd get a siege), or one side has to be outmarched (otherwise it withdraws, and we have a siege again).