Walmart has put out their shipping schedule that you can look at to see when you are able to order by to get things in time for Christmas. It is going to be pretty much the same for all stores, with the exception of Amazon that can usually get things to you a little faster.
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I would suggest setting a date to be completely done shopping by and sticking with it. There is nothing worse IMO than frantically trying to get all the last minute gifts and missing out on spending time with family fighting for a parking spot at a store. They can live without it to save yourself some stress.
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