It's the closing date - not when you tender your shares. When you deliver the shares and get the cash is when it becomes a taxable event. Right now it's just an accepted offer, similar to a real estate transaction - its the close date when cash and property (equities in this case) are exchanged.
Thanks wallstarb. That's good to hear. The PR does state the transaction could close as early as 12/30 so I guess I'll have to keep my fingers crossed that it takes a few extra days.