Hi and welcome to iLounge!
You can actually just drag-and-drop the songs to another directory, directly out of iTunes. This will copy them to that folder, and you can then delete them from your iTunes library.
For example, if you have all of your Christmas music in a genre called "Christmas" then you would just select that genre in the library browser, then do a select-all on the tracks (Edit->Select All), and drag and drop the highlighted group to another folder in Windows Explorer or Mac Finder.
Then, in December you could just drag all of those files back into iTunes and they would be re-added to your library.
Note, however, that using this method will remove any ratings and playcounts that may have been associated with those files. If you want to preserve that information, you'll basically have to leave the tracks in your iTunes library.
Something else to keep in mind, however, is that if your primary concern is the get them off your iPod itself, then you can actually just UNcheck all of your Christmas songs and set iTunes to "Only Update Checked Songs" in your iPod preferences (iTunes/Edit->Preferences->iPod). This will basically remove any UNchecked tracks from your iPod, and will not put them back on unless you REcheck them or turn off the "Only Update Checked Songs" preference.