I know you think this card is a few days late.

I promise this card isn’t late. 

It is intentionally arriving after your birthday because 

I think you deserve a whole birth-month.

 Happy Birth-month!

 I know you think this card is a few days late. 

 It’s actually worse than that.

 This is your card from 2010 that I only just got around to sending.

 Happy Belated Birthday!