Stacy and Nick went all out with a Batman-themed house! You could modify their procedures to make a custom sign and projector image for any holiday.
Arkham Sign: Make a template - you could use Adobe Illustrator software to print your template out in tile mode. Trace your template onto a styrofoam-like surface - Nick used a sheet of foam insulation from the hardware store. Next, cut your sign using foam-carving knives (the kind used to carve foam pumpkins). Finally, spray paint black. Be sure to use acetone-free spray paint or it will eat through your work (spray-test a small corner first). Even regular spray paint will eat up the foam a little. An alternative is painting the sign with tinted primer (tint regular primer with black paint) and then painting it black. Afix your sign to PVC pipe and secure pipe to the ground by sliding it over embedded rebar.
Easy Way: Use a high-powered lantern and foamcore board. Cut your design out of the foamcore and place in front of the lantern. Black out the other three sides of the lantern.
Nick's Way (uses a projector, music and DVD authoring software): He made an Adobe Illustrator file of the bat signal and laid it down on a video track in Adobe Encore*. Then he placed his music (the score from the first Batman movie) on an audio track underneath it. He added a jumpback action at the timeline's end to go to the first chapter marker, thus keeping the 13-minute long video looping. He burned his new DVD and played it through a home TV projector via a DVD player that was contained in a cardboard construct. The player and projector were both set on a stool and plugged into an attached multi-adapter.
*Note: Adobe Encore is a DVD authoring suite that comes with the CS4 Production Bundle. You could do the DVD step with iDVD on a Mac or the Windows equivalent.