As per your question I can think of three options:
1. Send all your regulars (or semi-regulars) pm's or emails and make the offer.
2. Post an ad asking anyone who saw you to contact you for a special thank you. Some who didn't see you may sneak it, but, worst case, it's new clients.
3. Post an ad to celebrate with a hot special (less hassle than #1, maybe more new clients than number 2).
Lovely photo by the way!
Regards,