Generally, unless something unforeseen happens, they will enter the .ocks pretty close to that time. Ships have to schedule appointments to transit the locks. This allows the canal authority to get everybody all lined up so transits happen smoothly. Can you imaging the chaos if ships just showed up and jostled for position?
On your first passage through the locks, be sure to get a place near the bow of the ship. It is really quite amazing. (Hmmmm. I need to schedule another Panama Canal cruise. Haven't been on one in quite a while.)