You can sail off as soon as the check-in with the charter company is complete. But if you arrive late in the evening, leaving the same day is often not possible.
Check-in happens in daylight
The boat handover and inspection take place during daylight, so they can be done properly and safely. If you arrive late, you complete the check-in the next morning.
Staying in the marina overnight
If you arrive late, you can sleep on board in the marina. The berth for the first and last night is often included in the price.
Leaving right after check-in
If you want to set off on the first day, pick an anchorage in advance that you can reach within a few hours of the base marina. Do not sail in the dark, and plan your arrival time so you get there in daylight.