Possible Clues:

• Busy time for Hallmark
• Present-day Roman coin in evidence after thousand-smacker return!
• Sounds like you've got ten mothers here
• Yuletide, slangily
• 'Do not open 'til ___'
• December ad word
• 79 Across time
• Winter festival marks cracking return of jazz player
• Brief December celebration
• 16-Across, briefly