A Financial Times article entitled How do I get my staff to kick their post-holiday blues? quoted Rita McGrath:
Post-holiday let-down is emotional and requires an emotional response. Break the re-entry into three stages: catching up, letting go, and moving on. The catching up stage for many employees can induce despair. Ease the burden – declare an e-mail moratorium and hold off on more assignments. It deals with the work and shows you care.
In letting go, the task is to celebrate the vacation but get people focused on the here and now. You might consider asking people to reflect on something they learnt on holiday that might have business implications. The moving on phase is your opportunity to get people focused on interesting activities. It is easier to leave the holiday behind if you have something exciting to look forward to.