She doesn't know how he did it.

Two or three days before an event he'll remind her every waking minute till it's been put upon a billboard with flashing lights in her brain and makes sure those lights are always .

He had some kind of weird memory where he remembered everything; their one year anniversary, their five hundred day anniversary, Valentine's Day, her mum's birthday, his mum's birthday, some friend's birthday or wedding. 

Public holidays, her birthday and his birthday (although she forgets whether it's the seventh or the eighth of February) are all the dates she remembers.

No more, no less.