"So, all in all, not one of Ron's better birthdays?"
--Fred Weasley, (HBP19)
Ron wakes up to an impressive haul of presents, including a wristwatch from his parents. He accidentally eats Chocolate Cauldrons that Romilda Vane spiked with love potion, and falls madly in love with Romilda Vane. Harry takes him to Slughorn, who whips up an antidote. To celebrate Ron’s birthday, Slughorn opens a bottle of mead, which turns out to be poisoned. Harry’s quick thinking and a bezoar save Ron’s life, but Ron still ends up unconscious in the Hospital Wing.
These events lead to a reconciliation between Ron and Hermione, and Ron realizes he no longer wants to be Lavender’s boyfriend, but can’t work up the courage to tell her.
This is the first time we see Ron or Hermione's birthday celebrated in the books.