From the Super Mario Wiki
To access this level, the timer must end with the two digits "33" (that is, "x33", where "x" can be any number) when the player reaches the flagpole of any World 3 level. Once the level is completed, it can be accessed again by satisfying the above condition once more.
Mario begins on an island in front of the pipe. Entering it will shoot Mario to the sky. He will first land on a moving yellow platform, where coins begin to shoot up from below like a fountain. This is followed by a Roulette Coin Block. More coins begin to be shot out where two 10 Coins and one giant coin fall from the top right corner of the screen. This is followed by two Roulette Coin Blocks and one large Roulette Coin Block. Finally, more coins will be shot out, marking the end of the level.