Start your new year resolution now and not wait till next month, but manage your own expectation realistically.
Don’t over-shoot and end up giving up earlier as a result of failure.
On a related note, don’t stop doing what you have achieved so far this year, for example, going to the gym and such.
If you are busy with the festive activities, ease out your routine but don’t stop completely.
It is very difficult to pick it up again comes January. It would be a double whammy in that case.