Dave already mentioned the previous thread for days appropriate to wear a kilt on, but some of the "biggies" are:

January 25: Robert Burns's birthday

April 6: Tartan Day (in honour of the Declaration of Arbroath, 1320)

April 16: The Battle of Culloden, 1746

June 9: St. Columba's Day

June 24: The Battle of Bannockburn, 1314

November 30: St. Andrew's Day

December 31/January 1: Hogmanay/New Year's Day

Cheers,

Todd