I doubt it is because of civilian pay because they are salaried, not hourly. IOWs they are going to be paid for that day at the same pay rate whether they work or not. There is no holiday pay rate for them.
I would not look for any deep meaning. For all anyone knows is Hagel said he can't do it on Wednesday, or the Thunderbirds have an air show in Germany Memorial Day, and since they are going to be overseas the weekend prior they want an extra day to guarantee they will be there....planes have a habit of breaking.
Honestly, I would think those two reasons are more likely than civilian pay costs.