You will get mixed answers on this. Many say you should stock Christmas all year round because people shop all year round.
I started stocking on July 1st. Mostly because this is 'down time' and I'd rather stock up now while I have a chance, than deal with the stress of Christmas on top of creating new items.
Christmas in July is a retail marketing strategy of generally having sales supporting early Christmas shopping during the summer. I 'celebrate' by just stocking Christmas items, but some people host sales, or advertise Christmas gifts! I believe the 'official' CIJ started on the 11th of July, but I could be mistaken. There is a team on Etsy to help with CIJ sales!