I'm going to Belgium on April/May, and wanna know if most attractions opens on May 1st Holiday. I'm planning to be in Bruges or Brussels on this day.
The popular tourist venues will be open and you can expect market stalls in the central plazas selling refreshments and souvenirs (weather permitting). Expect queues.
Restaurants and bars will be open in and around those venues. Cathedrals will be open. National rail will also run.
What will NOT be open are banks and any other financial concern that needs to execute or clear through the National Bank of Belgium or the Central European Bank (which essentially means all banks in Europe). Also, shops not relying upon the tourist trade will generally be closed. Bureaux de Change will be open. Cash points will also be open.
Note: this answer works for the early and late May holidays only. This answer will decidedly not work for Christmas day and New Year's day.
I my mind this is a fairly broad question, as opening hours, and hence closing days, are often not common across tourist attractions. Moreover you have not specified which attractions you are interested in, rendering it even harder to compute a list of opened and closed places. The answer to your question would therefore be: some attractions are opened, others are not. For example, the Atomium should be open, whilst the Royal Museum of Fine Arts should be closed.
What I suggest is for you to look at the Brussels City website, where you can find a list of tourist attractions, and their opening hours by navigating to their respective websites.