I've only had one "off plan" day so far in the 3 weeks I've been doing SW.
I shared a combo platter of deep fried goodies with one of m best friends, and had a few cocktails.
I don't think I've ever had a 'flexi day'. If I know I'm going out, I'll save my syns for the occassion. For me 105 syns in 1 day is enough and I have been known to have 70 syns in one go