White Rabbits for the first of the month goes back centuries, when people were a lot more superstitious than they are now. It's was thought to bring good luck if it was the first thing said upon waking.
I suspect it goes back even further than the wiki entry. It might have origins in hare lore. Hares were sacred to the moon goddess and as before there were months there were moons it was likely that to see a hare at a particular moon phase was considered a good omen