Day 1 is extremely important and each day 1 is different. For example, would day 1 begin when the deal is signed or would day 1 begin after the deal closed.
Based on my experience, day 1 is important to let the acquired company know what the acquisition means to their business strategy but more importantly what happens to them. “Me issues”. With employee retention, a key risk in a lot of acquisitions, it is essential that day 1 focuses on giving the “Target” company’s employee information on what to expect in the incoming days and what will happen next. Communication is key. Similarly to Dale Deg’s comment, day 1 messaging is extremely important as well. It is important that the target company messaging mirrors the acquiring company.