There is no fast rule and it all depends on the individual. Some take down their decorations on the day after Christmas, others after New Year's Day. Some wait until after the traditional day of Epiphany (January 6) which is considered the 12th day of Christmas. Yet others do not bother to take down their outdoor lights and leave them up year round, much to the consternation of their neighbors.
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Amazon.com is a good resource for these decorations. Consider searching for The Nightmare Before Christmas decorations, or asking for this in stores, as the movie blends Halloween and Christmas very well.
You can find ideas and instructions for handcrafted Christmas decorations on this site (http://familyfun.go.com/christmas/christmas-decorations/christmas-ornaments/). You can make anything from homemade paper stars to candy cane signposts.
Oftentimes Christmas decorations will be on sale before the "main" Christmas season so you may find good deals at Halloween time. Usually peak time is when they will be most expensive.
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The tradition in the US is that the Christmas season begins as soon as Thanksgiving is over. But it's really up to you, when (or if) you want to put up Christmas decorations. The religious police will not arrest you for failure to decorate on schedule.
It is believed to be okay to put up Christmas lights a little before or after Thanksgiving, but this is still more of a personal choice. Most stores put out Christmas decorations right after Halloween in preparation.
Many people begin shopping for Christmas decorations very early in the year. Because Christmas is such a huge holiday, most people like to spread out the expenses over the course of several months. It actually helps the customers to have decorations out early to help with budgeting.
I like shopping at Kmart, Target, and Walmart for outdoor Christmas decorations. However, their stock may not be out yet. Check their online counterparts or Amazon for some ideas.
Austria decorates their Christmas tree with fruits, nuts and glass ornaments, and they hang up their decorations four Sundays before Christmas.
You could do a 'Nightmare Before Christmas' theme, like the movie, and use all your scary Halloween decorations for that. Or, instead of buying orange colors, buy black and white Halloween decorations and use the white ones for Christmas.