There is always a time where you go along, do something, day something and it means nothing to you. No warning or reminder floats in your head that maybe you need to be saying or doing something else.
It's like your brain is cruising in an ocean, and suddenly you get sucked into a whirlpool and it is only just before you take your first breath of sea-water that you remember the signs that you paid little attention to a few nautical miles back.
This is not such a drastic situation, but nevertheless...
Happy birthday, for yesterday. Sorry.