I live in Brussels, however I haven't gone to the regular salsa parties here in a while, so I cannot really comment on the current situation regarding the level of dancing, number of people etc. But the following parties will take place in that period:
Wednesday 22.05.: Fiesta Salsa K-Nal - 5 € - K-Nal - Avenue du Port 1 / Havenlaan 1, 1000 Brussel (free workshop from 19 to 20.30, party is till 1.30 am). For the music: "salsa love, salsaton, timba, salsa moderna et de Bachata" (from event description on Facebook) :/
On Thursdays there's usually "Mambo Time..!!" at Mantra Bar. https://www.facebook.com/events/279883035440813/?fref=ts, however, for some reason the agenda on www.salsa.be says it will be on Friday, 24 May. So it might be a good idea to keep an eye of the FB event.
Usually they play good music there (mostly mambo + some bachatas and kizombas every now and then). I haven't been to this party since January, but I'd say it's the best weekly salsa party in Brussels.
Then on Sundays there is La Tentation, Lakensestraat 28, 1000 Brussel. But I have been there only once 2 years ago and didn't like it, because there were a bit too many people who were looking for something more than just dancing And the music was mostly Cuban then, but now it may have changed.
If i hear about other events that might be interesting, i'll let you know. Also, if you have more questions, don't hesitate to ask