From the Super Mario Wiki
To access this level, the timer must end with the two digits "77" (that is, "x77", where "x" can be any number) when the player reaches the flagpole of any World Mushroom level. Once the level is completed, it can be accessed again by satisfying the above condition once more.
Mario begins on the floor of the world beside a Cannon Pipe. The overworld music here is a remix of the overworld theme in World 1. As Mario enters the Cannon Pipe, two rows of coins appear one after another above the pipe and Mario gets shot out into the sky, collecting the coins. Mario will then land on an orange moving platform. Coins will then begin spewing out from the top and bottom of the area. Three fast spinning coin rings of various sizes can then be found and collected. A large Coin Roulette Block containing 10, 50 or 100 coins can then be found. This is followed by another set of spinning coin rings and a Red Ring. The Red Coins spin along with several spinning coins which has a flower-like pattern. Collecting the eight Red Coins replaces the player's inventory with a Gold Flower. This is followed by more coins being spewed out from the top and bottom of the area. Finally, a Giant Coin will fall, marking the end of the course.