Do you need a reservation for the dat trip for any day or can you get them the day before?

I recommend that you book in advance.

We were told they are booked through May. Book way in advance

I booked mine a year in advance. Upon arrival there were groups who had tried to make the standby list three days in a row and failed. I would definitely recommend planning a few months to a year in advance.

Same as others said for reservations, but we were one of the lucky ones who got there early. We arrived at 5am and were 2nd in line after a couple that arrived at 4am, and 10 people were waiting by 5:40. That morning they let 6 of us on at 7:30 due to no-shows. One lady who got on with us showed up at 5am the day before as well, but everyone showed up that day and no one got on from the stand-by list. Another day 10 got on, it's entirely hit or miss.

Reservations are booked weeks in advance. We scheduled ours 3 months before going.

Reservations are booked full weeks (sometimes months) ahead of time. If someone is a "No-show ", you can wait there and 10 minutes before departure you may be put on the boat

Yes, a reservation is best. They often book up months ahead of time. There are a lucky few who have arrived very early in the morning on the "day of" and been able to catch a cancellation and walk on.