How are you defining “Day 1” – literally the first business day following the completion of the transaction?
If so, I would treat it similarly to the first day of starting a new job – communicate, communicate, communicate. Yes, there’s absolutely a place for celebration on the first day, but from the perspective of all of the people who are now wondering whether they still have jobs – articulating the integration plan and letting them know that this is how they fit into the larger organization (assuming that this is true!) seems like something to start sooner than later. The longer you wait, the quicker gossip fills in the gaps, and the harder it will be to un-stick perceptions that have gained traction without any factual basis.