What do you mean by they won't celebrate custom holidays? As long as you are controlling a family, they can celebrate it. Now, I'm not an expert, so you may need confirmation, but as far as I know, I don't think a mod like this exists, nor do I think it would be advisable. Because when you are not playing a family, the sims are "suspended" for lack of a better term to save memory and to cut down on the processes (as someone who loved Sims 3's open world, I can tell you that it put a heck of a strain on a computer).
Now, unless I'm mistaken about what you mean by celebrating of course. If you set your calendar and holidays to have the decorate tradition, you'll at least see the spandrels and little flags on the other houses in the world, but other than that, that's about all you can do, as far as I know. Sorry if this doesn't make sense. I should be heading to bed XD
