Our big server upgrade is coming up in just a couple of weeks. Our intrepid network administrator has chosen Christmas week to take down the old servers and bring up the new servers in their place. Exchange is projected to be down for a couple of days, the admin servers for the better part of a week, and the academic side of the network for at least a week, if not longer. Why perform a server migration halfway through a school year? It appears to be the time when the least users are active on the systems. There is actually quite a lot of administrative, teacher planning, and student email activity during the summer. A network migration would be more disruptive then than it will be during Christmas week.
Of course, the pressure will be on us to bring the servers back as quickly as possible. Note that we made this more difficult for ourselves by choosing to change our network structure. If all we were doing was to upgrade the servers, then the migration would be a lot more seamless, involving only data migration.