Never hurts to mention it ahead of time and again at checkin. My wife's birthday was last week, and I had booked two nights at a Hilton we often visit. Mentioned her birthday during our stay in the comments section of the booking, then again in an email to the hotel the day before. We were welcomed at the front desk by a smiling "happy birthday" when we arrived, and were put in an upgraded room with robes and slippers for both of us, followed later by a knock on the door and a birthday card signed by the front desk staff and a small jar of candied strawberries and blackberries. Later on that day a delivery of chocolate liqueurs and large bottle of Voss carbonated mineralized spring water from Norway and another card from the front desk manager.

Other stays for our anniversary have resulted in chocolate truffles and upgrades at another hotel, and a large chocolate mousse for our anniversary at another over the years.

So don't be afraid to mention the occasions. Sometimes it can make the difference in your stay.