Royal Mail issued the reminder online, by publishing a letter they 'received' from Father Christmas, urging kids to get their letters in the post by 6 December.
Each year, 800,000 letters are sent by children in the UK to Royal Mail's special address: "Santa/Father Christmas, Santa’s Grotto, Reindeerland XM4 5HQ".
They are then answered by Santa* himself (*postal workers and/or outside volunteers), but only if the letters are sent off in time.
According to the Telegraph, a reply is sent from a sorting office in Belfast.
The note from Father Christmas reads:
"Dear boys and girls,
"Christmas is a very exciting but busy time for me: the elves are making toys, the reindeer are practising their sleigh runs, and I'm busy preparing my list of who's been good this year.
"I need to let the elves know what to make so to help me out, please let me know what you would like as early as possible.
"I will be getting my sleigh ready for the long journey on Christmas Eve and in between I will try to reply to as many of you as possible. Please send your letters to me by Sunday 6 December so I have time to reply."
All children sending letters to Father Christmas must write the address carefully on a stamped envelope and include their full name and address so he can write back.
Chief Elf Alex McConnell said: "Royal Mail plays a special part at Christmas.
"We are proud that for over 50 years Santa has allowed us to help manage the thousands and thousands of special letters he receives from boys and girls across the UK.
"His special team of elves at Royal Mail enjoy taking a peek at what children are asking for, and helping Santa reply to these special letters."
Check out some brilliant letters to Santa for some inspiration in the slideshow below.