There is a set announcer in charge of each day of the week.
The announcer in charge of each day of the week is decided.
「決まってる」→「Set, decided」
「アナウンサー」→「Announcer」
「曜日」→「Day of the week」
「担当する」→「To be in charge of」
まとめて言いますと、
「There is a set announcer in charge of each day of the week.」
「The announcer in charge of each day of the week is decided.」
と表現します。