Umm....
Agree with @Tim's logic, however...
My take was that "Singles" is a (mass?) noun describing all single people - after all you don't call yourself "a single".
Also, when we have a day celebrating, say, books - it's "National Book Day". not "Books". However "National Single Day" sounds daft.
So...... maybe no apostrophe?
Biting the hand that feeds IT
