Very appropriate to give him a send-off party. It doesn't really matter if he is going to still be in the district, he is leaving your school and the kids will likely not see him again.
Sure you are in a time crunch but I'm sure there are some that are willing to put some time in for this.

It sounds like he is leaving his position as Principal, so I would think he deserves a send-off. I probably wouldn't call it retirement, but that's a minor detail.

We are in an odd situation. Our beloved principal had put in his retirement papers last year with the intent to retire at the conclusion of the 05-06 school year. Now he has accepted a newly created position of district elementary coordinator, with the intent of still retiring end of next year. We thought we had plenty of time to plan a retirement party, but he will be leaving our school in early January (and not actually reitiring-yet)

Does anyone have any ideas of what to do for a "retirement/promotion" type of event? Would it be appropriate to have a retirement assembly with the kids although he will still be in district (with an office in our building)? My mind is blank and am looking for any suggestions or ideas. Anyone?