Refers to the daytime portion of an event or schedule, especially a matinee performance. Often contrasted with 夜の部 (evening session).
今日の公演は昼の部と夜の部があります。
Today's performance has a matinee and an evening show.
昼の部のチケットはまだ残っていますか。
Are there still tickets left for the matinee?
Compound of 昼 (daytime) + の (possessive particle) + 部 (part/section).