We can split up what we have to do on Wednesday into three, big things.
Every Wednesday after school I go to the pool and swim.
He got drunk out of his mind Wednesday night and was run over by a truck.
This is a day of the week. You know this because it has 曜日 in it. But what day of the week is it? The water day of the week is where the water flows from. Think of your entire week as a big hill, with Wednesday (hump day) being in the middle. This hump is a hill. What originates from hills? Water! That's why Water day is Wednesday.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. It will help to know the word 曜日 as well.