Re: took the day off
Sorry, no. It's not normal at all. As the OP suggests it is actually incredibly bad planning. If either a PM or TM had been that lax on my watch, he would be out of a job for allowing such a basic oversight.
On Operational day #1 after a major migration or implementation you should be in a Hypercare phase or similar, whatever you want to call it - where ALL key support and admin staff for critical applications are available on a rapidized reponse basis to quickly investigate all issues that arise that may prevent business operations continuing.
To allow the sole admin or support contact for (what sounds like) a mission critical business application to be OoO on the first day after a major migration or implementation is a schoolboy resource planning error that should have been caught by your pre go-live checklist or G/NG meeting, and then captured as an issue to be impact assessed and agreed with stakeholders at the final Y/N before the migration started.
It just sounds like shit planning, as the OP suggests.
Biting the hand that feeds IT
