There are similar events like "the night of museums" or the "movie parade" which try to draw the attention of people to certain topics, services or issues. The main idea is that for a really very cheap ticket you can enter more then 100 fitness clubs using their services for this special weekend.

The price is 500 HUF (apr. 2,5 USD) which is really very cheap. Last time I bought a one-time ticket in a gym it costed like 900 HUF.

Unfortunatelly the event has only Hungarian website, I havent find any information in English (it seems they haven't count yet with thousands of expats living in Budapest) but if you decide to participate you can check the list of the clubs here.