Hi there,
Day 1 is a critical milestone in the integration process and as such I also believe you should celebrate it pending of course of its success. Especially since D1 preparation takes many weeks (8-12 according to the syllabus) involving both the target and the acquirer, celebrating this milestone would help create a sense of achievement and bonding, which in turn may reinforce capability, focus and motivation for the next important milestones such as the 30 days, 100 days, etc…
