Semi Formal ideas for an Anniversary party

I would put a jacket and trousers together without tie.

In fact, if the parent's view of you matters, better to go with a suit without tie

Dress nicer usually attracts better treatments.

Not sure if they know the strict definition of semi formal or if they are referring to formal as suit and tie and semi meaning ditching the tie/no jacket/etc.

I would wear a darker suit, white or subtly patterned shirt, a pocket square that ads some flair, and keep a tie with you. You can either wear the tie OR open up the shirt for a more relaxed summer evening look.

I'd just ask the young lady what "semi-formal" means in her family.
Some might consider that to be "pants that aren't jeans and a clean shirt" while others think a dinner suit is semi-formal.

Haha, that just reminds me back int he days for British college, where I need to wear a tux for everyday dinner and a lot of white ties each month, good times

http://www.styleforum.net/t/149415/is-this-outift-ok-need-semi-formal-attire-and-im-clueless