This is like saying, "Wake me when it is time to get high."
420 is slang which is widely understood to imply ingesting some cannabis, marijuana, weed or cheeba-cheeba.
There are some who will not get it, but those tend to be the people you do not want to know about your interest.
Wake and bake.
Bumper stickers are not just for cars anymore. These can go many places, and make fine gifts for you-know-who.