The official date for Loy Kratong is 17 November but in Chiangmai the festival will be held from 15-18 November. Besides the Loy Kratong there is the Yee-Peng festival (actually they are both the same) with the floating of lanterns (Kom Loy) in the sky with the traditional happening (free but very busy) usually held the weekend before Loy Krathong and the commercial (80-100 USD - and not so busy) held a few days after Loy Kratong. However exact dates for the latter two (traditional and commercial) have not yet been announced.
However, during Loy Kratong there will also be hundreds of Kom Loy floating in the sky so I guess your best bet will be 17 November.