The special noun 予定（よてい）is used to tell somebody about a schedule.
21日に帰ってくるよていです。
I’m due to come back on the 21st.
Note, even though this word is often written in hiragana when used as a verb ending, it is usually written in kanji when used as a noun.
明日の予定はなんですか。
What is tomorrow’s schedule?