A War Move is one week. 8 days that is. Thus in one month there is 4 War
When you do a Declear War action, you thus get 4 War Moves to play with.
In a War Move you can do one of the following:
* Move your units (Up to their move),
* Occupy something (general rule is one holding per 3 holding levels, and 1 per 2 province levels)
* Occupy and reduce something (one unit reducing one level of holding per war move).
You cannot do the two second options if the units have been involved in something else. Like moving or fighting that warmove.
Fighting happens when two different armies occupy the same province, and neither side have said anything about not fighting the other/is not previously allied. Armies usually have the standing order to protect against outsiders who're not supposed to be there etc.
When fighting starts, the fight goes on for 3 full "phaces". Each phace is two full days of fighting. In each phace there is a move, attacking, defending and spellcasting. Battle Spells usually either take a full Phace to cast, or a war card round. In any phace there is 5 war card rounds. For a total of 15 war card rounds in a War Move (thus on the battlefield all move, fights/defends/cast personal spells 15 times).
Many spells last so so many rounds. So a Spell lasting 10 Rounds, lasts for 2 Phaces.