The Northwest Montana Fair (406/758-5810) is held at the fairgrounds in Kalispell [1] in mid-August every year.
If you’re in town during the concert season (fall through spring), catch the Glacier Symphony and Chorale (406/257-3241, www.glaciersymphonychorale.org [2]), which performs at the high school auditorium; they also play a summer pops concert in Bigfork [3].
Links:
[1] http://www.moon.com/destinations/montana/missoula-and-northwestern-montana/north-flathead-lake/kalispell
[2] http://www.glaciersymphonychorale.org
[3] http://www.moon.com/destinations/montana/missoula-and-northwestern-montana/flathead-lake/bigfork