Please feel free to leave anytime tomorrow afternoon.

Literal

Tomorrow [possessive-の] afternoon, anytime honorable-leave please.

お出かけください is the sonkeigo (honorific) form of 出かける ('to go out') — the pattern お + [verb stem] + ください is a productive way to turn any verb into a polite imperative. 'Please [honorable verb].' いつでも adds 'anytime,' emphasizing flexibility.