I usually start properly in July/August, but I. terms of stocking fillers I buy throughout the year.
When I was growing up I always got knickers socks, nice toiletries, a book, etc so I've carried this tradition on with DD. She's only 6 so I know I'm fairly safe with things like I had growing up as well as colouring books, stationery, hair accessories and so on.
Adding a couple of items in with the weekly shop from late summer lessens the financial impact come December, but if I saw something that I thought she'd like I'd buy it whenever.
We save for Christmas throughout the year so there's always money there in case we spot the perfect gift for someone. Ours is always bought and wrapped up by the end of November ready to distribute when we see family and friends in December.
I restock our cards, wrapping paper and Christmas crackers in the January sales and add to the decorations if I spot anything I really like - I picked up a couple of lovely Christmas throws on Christmas Eve, they were reduced to £10 (were £20) so I bought the last few ready for next year. I really liked them, but would have been a bit 😳 at spending £60 on them, whereas £30 felt like a bargain.