I have work on weekdays.
Sadly, I probably won't be able to go because it's on a weekday.
I am a chef but because I’m busy working on weekdays I usually don’t cook at home.
The flat days are boring. They're the same. One day after another. That's definitely not your exciting weekends... that's your boring, flat weekdays.
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. Note that 日 in this one uses its じつ reading.