Translation:What is the date on Sunday?
The connection of multiple pronunciations and meanings to the same characters, in addition to using multiple characters to describe the same meanings and sounds, makes the process of learning this language unbelievably frustrating. How does one go about recognizing what the symbols are supposed to mean when they can mean five different things and be spoken four different ways (each overlapping with the meanings and pronunciations of other characters of course) each??
They generally only mean a couple different things. 日 represents both "sun" and "day," so it being used twice in "Sunday" makes a lot of sense.
Although most kanji only have a handful of pronunciations, I find it easiest to learn what they represent and then to learn the vocab in which they're used almost as separate units. 日 might be に～ or にち, but I don't focus so much on that, and focus more on stuff like 日曜日 = にちようび.
English has similarly difficult things, like "set" having literally hundreds of uses, or "right" vs. "write" vs. "rite," or crazy pluralizations, or verbal phrases constructed with a verb and preposition, etc. Every language has difficult stuff.
https://jisho.org/search/%20%23kanji%E6%97%A5 According to that site, there are 6 ways to pronounce: hi, -bi, -ka, nichi-, jitsu. It could be pronounced differently when it's used to write name (see Japanese names section on that site) 日本 is an example. 日 is pronounced as に there, which isn't listed on 6 pronunciation above. My suggestion, I've tried to memorize Kanji, all of their meaning and every pronunciation. It's very hard. It's much easier to learn the phrases first then identify what Kanji used in the phrase (including the sound).
So while I can't fully explain it, I think I can give you at least a little bit.
The kanji, as you probably know, came from China. As such, they retain the 'original' pronunciation, the 'kun yomi', but they also have a new pronunciation, the 'on yomi'.
There's no hard and fast rule to tell you which one to use; it's mostly going to be a memory thing. In my experience, though, the kunyomi is more likely to be used when the character functions as part of a greater construct, and the onyomi is more likely when the character is still an 'entity of itself'. Vague, and possibly not at all correct, but it helps me to understand why day as in 'day of the week' is bi, and day otherwise is nichi