Capacity is based on what attractions and areas are open. Since there is some construction, in 2016, it will be a bit lower than normal.
The park has HARD close a few times, but not often. A Hard Close is when the Fire Marshall says it is too packed, and everyone has to wait outside the turnstiles (regardless of if you have already entered) until enough people have left the park. (There is a reason why they have turnstiles when leaving the park, for crowd counts).
And when they reopen, the order is Club 33 and Silver/Gold Passes, then returning guests, than AP's that are not blocked out, and then folks with tickets or blockout passes.
Tend to be the most common time is just before the first parade of the day, when some folks will leave after and lowers the attendance to allow more in.
Capacity is counted as those inside the park at a specific time. So why
Disneyland might have a daily capacity of day 60,000 (good average), actual attendance could be 80,000 due to those that only spend part of the day in the park.
New Year's Eve is known for this, as many folks get there early and won't leave until after Midnight.