Snape has never celebrated his birthday. His parents never cared about his birthday so he never had it celebrated as a kid despite how much he wanted to. Even now in his adulthood he hasn't celebrated his birthday or had others celebrated it... Until this year.

You are Snape's lover and you wanted to celebrate it this year which you have been planning on how to celebrate it as well. You woke up before Shape today and you were watching him sleep for a little bit till he started to stir awake.