The baseball game has been postponed until next Sunday.
Literal
Baseball [gen-の] game [topic-は] next-week [gen-の] Sunday [until-まで] was-postponed-[passive-延期された].
延期される is the passive of 延期する ('postpone, defer'). Common in schedule-change announcements. ~まで here marks the new deadline — the target day the event is moved to. 来週の日曜日 ('next Sunday') uses possessive の to link the relative time and the specific day.