Gary Jones

Can add multiple calendar widgets – each of them set to different categories (or All Categories).
The tooltip on the per-day links shows only post titles from that category, and when you click on the calendar link to bring up the day archive of posts, only posts from that category are shown.

Calendar Category

Un-registers the default Calendar widget, and registers a new one which supports showing only those posts in certain (or all) categories.

Marios Alexandrou

Ultimate Category Excluder, abbreviated as UCE, is a WordPress plugin that allows you to quickly and easily exclude categories from your front page, archives, feeds, and searches. Just select which categories you want to be excluded, and UCE does all the work for you!

Ultimate Category Excluder

Ultimate Category Excluder allows you to quickly and easily exclude categories from your front page, archives, feeds, and search results.

Calendar Category vs Ultimate Category Excluder