We have been there several times when the parks were full and closed!! Almost every trip was at Thanksgiving though so I would doubt that the crowds would be that big at the times you are going.
But....here are some tips and hints if it "looks" that way:
First of all, on a day there is a chance DL might close, there is a big giant "you can't miss it" sign right at the security check entrance that says, "
Disneyland is very busy today." If you see that sign going in, don't leave the park!!
Around 11 am, when DL closed, we were told the sign was changed to say "DL is full, enjoy CA" or something like that. They did
not tell people that were leaving that the parks were full and they could not or might not get back in!! Don't count on any Cm telling you or you will be disappointed. When we were there, people were practically fighting outside the gates because they were not told. One mom went to their car to get the kids sweatshirts and was not allowed back in, can you imagine!??!
We heard that people just waited in line and after some people came out, they would let some in. I was told there are several levels of closed: first they stop selling one day tickets for example, then no new entries, just people who had been there already(so if it is busy, make sure you get your hand stamped) and so on down the line until the fire marshall says "No one at all!!"
When the parks closed, it was always after at least 10:30 am so as long as you get there early, you won't have a problem. For us, the problem was we were afraid to leave and we were staying at the DLH just so we could take a break, swim and then head back. They told us when the fire marshall closes it, even resort guests were not let back in.
We learned to do some alternative planning just for the days we knew it might close. For example, we really really wanted to do the Thanksgiving Dinner at the hotel but decided instead not to leave the park that day and made ps for Blue Bayou. Instead of Goofy's for breakfast, we did the Plaza Inn so we would not have to leave the park. In the end, the park only closed on Thanksgiving Day(2&3 years ago) and one of the Fridays after so it was not too big a deal since we were there a few days.
On those crowded busy days, we really had to make good use of fastpass and know that it would be slow going with the crowds but it was all worth it. We were at DL!! You will have a great time no matter what but just pay attention to the signs. If you can not walk down main street without your feet touching someone else's feet, that is a good sign it is really crowded and if you get stuck in a massive traffic jam in front of Indiana Jones, that is another sure sign that DL is filling up-lol.