予定（よてい）だ is an expression that functions very similarly to
つもりだ ‘to intend to do’. The primary difference being that 予定（よてい） is a ‘plan’, rather than an ‘intention’. Due to this,
予定（よてい） is used far more frequently for things that you have formal plans to do (like meeting a friend for lunch).
To use
予定（よてい）, just attach it to the non-past form of a verb, or to a noun (after inserting
の).
-
今夜（こんや）は先輩（せんぱい）たちと飲（の）みに行（い）く予定（よてい）だ。
I plan to go drink with my senpai tonight.
-
休（やす）みは来週（らいしゅう）からの予定（よてい）だ。
My break is planned to start from next week.
If
だ or
です is not added after
予定（よてい）, it will imply that
予定（よてい） is simply modifying the verb/noun that comes before it, rather than being a standalone ‘plan’. This is similar to the difference between saying something like ‘I have plans to eat with a friend’ (natural), vs ‘I have an eating with a friend plan’ (unnatural).
-
明日（あした）は友達（ともだち）とステーキを食（た）べる予定（よてい）だ。
I have plans to eat a steak with a friend tomorrow. (Natural Japanese)
-
明日（あした）は友達（ともだち）とステーキを食（た）べる予定（よてい）。
I have an ‘eating a steak with a friend’ plan tomorrow. (Unnatural Japanese)