Without more context it is difficult, but maybe 100 degrees in temperature?

Thanks, guys! I appreciate it:)

Could it have been a very hot day? "Century" is used for a hundred; a hundred degrees in the Fahrenheit scale is quite hot, for lots of countries. And the Fahrenheit scale is still in use in the USA, I believe.

Hello! In my opinion, the sentences mean the following (but it depends on the context of what is happening in the movie, which is not clear in your question): "It's going to be one of those days." means that it is going to be a significant day in a bad way. "It's gonna hit the century mark by 11 am" means that the number of incidents that are happening in the movie will be a hundred by 11 am. A century has a hundred years in it so if someone speaks about a century mark it means a hundred. I hope that helps!