Both casinos in Sosua (Coral Reef and Playa Chiquita) are open, I believe the table games are available from 8pm onwards.
There are some Texas Holdem poker games that go on with about 500 to 3000 pesos starting buy-ins. I believe there is now a big game every month that you can play for an entry to by participating in games during some nights. I play sometime but it is really quiet these days in low season, same few people always turn up.
There are some Caribbean Poker tables, 3 and 5 cards. I play them sometime when I really want to loose money quickly !
The blackjack is 6 deck and 7 decks with 100 pesos min and 2000 max, favourable rules, not bad, but they cut the decks around 50% or 60% if you are lucky, so forget about counting. (I have no idea what other tricks they use, but I like BJ and play this often.)
Roulette tables all have 0 and 00 I believe - not my thing, I rather watch this one. And there are numerous machines where you just press the button and gaze at those rolling figures (never know the attraction, but they are usually busy).
As mentioned by others the casinos here are really not regulated and they do what they like, so like everyone else you are wise to just go for passing the time and not get too excited about real gambling and loose your shirt in the process.
Ask any other questions you have, I go 2/3 times a week, sometime just for looking around and admiring how casinos make their money !?
(Big warning for all, never play that Caribbean Roulette or whatever they call them these days, they are just scams - avoid at all costs.)