Historical clue formulations

• Ski-___ (snowmobiles) — 2013-11-17
• Ski-___ — 2012-08-23
• Ski-___ (snowmobiles) — 2010-08-10
• Ski-___ (snowmobiles) — 2001-07-12
• Whoop-dee-___ — 1998-08-29