There are 30 days a month. Ships move every X days, where X = 30 / movement rate. Watch the stepped movement log to see when all of those ships moved.
If you have formed a fleet, the ships in it move together, not separately. But, if you used shift to select a stack, they appear to move separately. Beware: the shift-selected ships only actually move together on the first turn of a movement order. After that, they move separately. That is why fleeting them is better: it removes some unnecessary micromanagement.
