You'll need to complete a few tasks before it opens, though:
* Complete the "Spring Festival" event. This happens on Spring 21st and involves completing a few mini-games.
* Deliver a certain number of items to the "Mysterious Person" in the town. This person will give you clues about the circus and what they need. You'll need to bring them a total of 5 items.
Once you've done both of these things, the circus will open on Summer 1st!