Jane, as Rosie said, get in touch with the hotel before you go and they will do something special im sure. Pop around the bars and you will be congratulated in allmost everywhere you go if you let the bar staff know. you will meet people who will want to help you celebrate your aniversary and I am sure you will enjoy!, post a report when you get back so everyone can catch up on how it went, Congratulations for the day!
Wear a DJ in Borovets and you're likely get a fish thrown at you
Seriously, you can wear what you like, but 99% of people dress down, that's the nature of the resort, and pretty much the nature of the sport.
Apres-ski is renowned for being informal and relaxed throughout Bulgaria.
A DJ will not get you thrown out of anywhere,, but it will attract some odd looks.
If you want to fit in with the crowd, just get him to wear a pair of trousers and an open neck shirt.