Possible Clues:

• Knee-slapper
• Scorcher
• Scorcher, say
• Sweltering summer day
• 100-degree day, say ... or each starred answer's end, based on a word that can precede it?
• Day for fanning yourself
• Dog day description
• Definitely a day to run the A.C
• Day during the dog days
• Definitely a day for air-conditioning