Dear Newcastlelou,
As you may already know the HbA1c test will show what is your average Blood/Glucose levels that you have had in the 120 days prior to the time when your Blood Test was taken. If you seriously intend to overdo the 'Christmas Period' you will naturally increase your reading from what it would have been by taking the test after the Christmas period, and having overdone all the 'Christmas Goodies' over the Christmas/New Year Period.
When to take you HbA1c test is obviously up to yourself so you will have to decide whether to go for it NOW and risk what might be a diabetic outcome or take a chance and hope for the best, remembering that if fate has set it that you are to join our diabetic community as a diabetic it is obviously better to know this ASAP.
Diabetes isn't normally confirmed by just the HbA1c test alone and a early morning Fasting B/G test is often taken to go alongside a HbA1c test to confirm if you are diabetic or not, but I would think that the 'Christmas Goodies' will notch up what otherwise would be a lower HbA1c reading.
The decision is ultimately down to you, but personally I would like to know sooner rather than later.