Would it be convenient for you to start work tomorrow?
Literal
Tomorrow work [object-を] begin [nominalizer-の] [topic-は] convenience [subject-が] good [tentative-でしょうか].
ご都合がよいでしょうか is a polite formula for asking about someone's convenience. ご都合 uses the honorific ご prefix on 都合 ('convenience'). The ~のは construction nominalizes the action and makes it the topic for evaluation.