The concept of the Uitmarkt is that the public gets to see small samples, usually 15 minutes, of the performances and shows of the coming season. On a large outdoor market, theatre groups get the opportunity to present themselves.
The sample shows are free, and can be seen in many venues and several temporary outdoor stages, almost all in the Eastern Harbour district (a map of the area is on the Uitmarkt website, see below).
The area is about a 10 minutes' walk east of the Central Station.

The opening show is on Friday evening, starting at 8.50pm, at the head of the Java Island.
Java Island (Java-eiland) is one of the peninsulas in the Eastern Docklands area. Bus 42 from the Central Station goes there, or you can walk there in about 30-40 minutes. See also this map.

The Uitmarkt will continue during Saturday and Sunday.
It is expected to draw half a million visitors

More info is on the Uitmarkt website (mostly in Dutch, with English summary).