January, nice! Didn't realise this would be out so soon, or that it'd be a 2 part film. Hopefully this one doesn't have a bloody French title again though, like doom that came to gotham..
Thankfully you don't need to watch 37 lol this is part of the "tomorrowverse", of which there are only 6 before this. I'd suggest watching them all, but you could probably get away with just the last one, warworld, as long as you have a decent understanding of DC characters and history in general.