When to board and what to do first?

Depends on the cruise line. Celebrity starts at 10:30a for suite and high level loyalty program members. For high end suites, often the rooms are ready. For lower end suites, they start opening up after noon-1ish. We always hit the martini bar first, then look for lunch.

If departure is 4pm, keep in mind that most (all?) cruise lines require being checked in at least 2 hours prior to departure. So, your window is probably somewhere between 10am and 2pm. If it were me, I'd probably try to miss the early initial rush. There may be a lull between the early rush and the late rush. That's my sweet spot.

If lunch onboard is important to you, also keep in mind that while at least one MDR should be open for lunch, it probably stops serving around 1:00 - 1:30pm. (The feeding trough, er, buffet, will likely be open until 3:00 or 3:30, or just prior to the muster drill.)

Some like us prefer to board early so we usually set a 10 am check in and board as soon as we are able to enjoy things on the ship and explore before sail away but others prefer to have a leisurely morning and board around 2. Depends on what works for you. But I do believe that you are required to board at least 1 hour before departure.

Each cruise line is different, but normally the boarding starts between 10am and 12pm. Then the rooms won't be ready until at least 1pm, but again, it may depend on your cruise line. You could probably contact the cruise company to find out more specifically when they want you to board.

https://cruiseline.com/forum/post/the-first-thing-s-you-do-after-stepping-onboard

There is a full thread about this topic here

Thanks. I really appreciate your answer. This will be my 2nd RC cruise. It's been probably 18 years! taking my 91 year old Mom and I know she will need her nap

Usually they start boarding around 11-noon. Rooms aren't usually ready until 1-1:30 pm. But it depends of course on the ship/line/how on time everything is going!