Dag Yoona, I've learned => adjectives end on an extra e standing before a noun, except when this is an indefinite (using een) het-word. This rule still makes sense in this case. The thing that happened here is that we have an indefinate noun that doesn´t take ´een´. this happens when you have a non-count noun. A non-count noun is a noun of a substance that you can´t count. with these nouns you don´t use an article when you use them in an indefinate sense. e.g. ik drink melk, ik adem lucht. The same thing happens in English. you don´t say: I drink a milk. you say: I drink milk https://www.talkenglish.com/grammar/count-nouns.aspx ´weer´ is een non-count noun. je kan niet zeggen 1 weer, 2 weer, 3 weer.. I think that the word order in the sentence ´Maandag was het mooi weer´ is confusing you. the ´het´in this sentence is not an article, but it is an subject pronoun. Maybe it´s clearer if I change the word order. Het was mooi weer maandag. In short: you have an indefinate noun that is an ´het´word'. this noun is preceded by an adjective. so you don´t put the extra ´e´. i hope this makes sense. Veel succes met je Nederlands!

The word order is a bit confusing: "Mooi weer" doesn't have an article. "Het" is really part of "het was" (IT WAS nice weather on Monday), it's just moved before "mooi weer" because "maandag" occupies the first position in the sentence.