One thing you will find on any morning in Cuttyhunk pond, is the gradual procession of boats heading out the pond enroute to the Vineyard. We typically layover there before heading to Edgartown as well, but usually leave by 8:00 - sometimes later, depending upon the crew.
I've witnessed several boats leaving later in the morning. By 12:00 - 1:00 pm, all moorings in the inner harbor are usually taken for the night ahead. We always find some entertainment in sea weiry late arrivals scrambling for the last moorings.
The outer harbor moorings fill up by mid afternoon - if not earlier. This area as you probably know, is subjected to northerly winds and wakes.
I would guess that during this weekend, moorings may be more plentiful due to forecasted showers and thunderstorms on both Sat. & Sunday - sure put a dent in our Block Island plans. So, you may luck out at any time you arrive there.
If anchoring is necessary, be wary of eel grass and shallow depths in the north anchoarge field. I've found the holding to be fairly decent - although we've only had to do it on one occasion.
Many cruisiers don't realize there are poles (or dolphins) available to the northeast of the pond - directly in front of the Cuttyhunk Yacht Club. I think they're maintained by Frog Pond Marine - but check the posted signs on each pole, which also note the fee - actually cheaper than the Town maintained moorings. I've never taken one - but there always seems to be one or two available.