I make a lot of mistakes in the readings of words with 日 in them, it’s hard to remember whether I’m supposed to read it as にち or as じつ. WK doesn’t even seem to acknowledge this difficulty, the reading info just says “this is the reading you learned with the kanji, you should know this” for all of these words, which is kind of infuriating!
In my experience you just have to memorize the readings, however, there seems to be some instances where one is favored over the other. For dates, and when you are talking about the daytime in general, you will usually use にち. じつ seems to show up when you are talking about a specific kind of day. For example, 休日 kyuujitsu - day-off, and 祭日 saijitsu - holiday. Frustrating, I know.
Don’t be frustrated. The more you use the language, the easier it gets to remember these things. There are no rules, and I wouldn’t try to rely on any particular trick because in the end, the goal is for it to just “feel” right because you’ve used/heard it enough.
What you can do is practice KaniWani and Kamesame to reinforce producing those readings, but other than that, keep going and it’ll eventually sink in. You’re early in the process, no need to rush.
What helped me was taking out a piece of paper and writing out a list of all the terms (up to my level) that include 日, writing out the reading and meaning for each. Then I sorted them by reading. I could see a pattern similar to that described by @AFMFG… I encourage you to do it yourself! You’ll gain more confidence and certainty.