How to Count Monthly Birthday?

Mel
This is probably a silly question but...
I'm wanting to take a monthly photo of baby to keep track of her milestones and keep my family on FB in the loop. 
She was born on November 13th and will technically be four weeks on Sunday (11th). Would it make more sense to celebrate on every 13th of the month or to follow it depending on the actual day every month lands on? 
What did you moms do? 

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