yes, you can. why not?

I think the biggest trick with discounts is not to make them predictable. If you run spring / autumn promotions people will wait to buy as they know it's coming, so its definitely a good idea from that POV.
Plus it's fun, human and an extra feelgood factor to celebrate. I saw Justin Jackson is doing a sale for his birthday which I think's great too - you're basically giving yourself a nice present on that day :)