Weather in poland is mild in october with lows of 3 and highs of 17, with a fair bit of rain. Depending on how formal the dress code is I'd go for a classical cut dress (eg prom, or shift) and either a cropped jacket or a dress coat.-So you'd look smart and warm/dry. Try: www.asos.com- dresses www.oasis-stores.com and at the moment I'd try debenhams as you could get a bargain in their latest summer sale! Enjoy the shopping and hope you find something perfect! And congratulations!