I always do my Pickled Onions about a week into December, then the Pickled Red Cabbage one week before the 25th.
I couldn't find any pukka pickling Onions so used Shallots instead.
After peeling they are covered in Water with a reasonable sprinkling of Salt. They stay in this solution for 24 hrs, then are washed, packed into jars, and covered in Malt Vinegar with some Sugar and a few dried Chilli flakes. If you don't like them too 'vinegary', add about 20% Water.
I do buy ready-made Pickled Onions, but they are never as good as home-made, and of course they are more expensive too.
They need to be left for at least a couple of weeks before sampling. I shall do one lot per week until the Turkey's finished. Christmas ain't Christmas without pickled Onions and Red Cabbage.