Possible Clues:

• Costliest production losing a little time: The Longest Day
• Midsummer or midwinter point
• 21 June or 22 December?
• Biannual turning point in star’s career
• Biannual event is close, changing with time
• June 21, say
• Time in June and December when the sun is at its greatest distance from the celestial equator
• It’s close at work —a special day
• As long as it's short, it's a turning point in June
• Note slippery patch on road: that is expected in December