A task action is something you have to do (because 事 is all about actions). What's another way to say "an action that is a task," though? I would say errand or even business.
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.
On that day, I have important errands.
Oh, Tanaka-sensei. What business brings you to Don Quijote?
Excuse me. I'm sorry to leave early, but I just remembered there's something I have to do.