Christmas is a magical time in Elton John and David Furnish's household since it also marks the anniversary of the couple becoming first-time fathers.
Sir Elton took to Instagram on Christmas Eve to pen an emotional message to his eldest son Zachary ahead of his 13th birthday, reminiscing about the moment in December 2010 when they found out their surrogate had given birth. The pair celebrated the special day by presenting young Zachary with a delicious-looking chocolate cake, with the words "Happy birthday Zachary" piped onto the tennis-themed confection in green icing.
The Rocket Man star wrote: "Where has the time gone?! Thirteen years ago was a Christmas Eve like no other. On the 25th, we received the best Christmas gift ever, with the birth of our beautiful son Zachary.
"We are so deeply proud of him, and love him to the moon and back. Happy 13th Birthday Zachary!!!"
Fans of the music legend couldn't believe how time had flown and began flooding the comments section with heartfelt congratulations.
"Still in shock he is 13, how did that happen?!?! Happy Birthday Z xxx," one wrote. "Happy 13th birthday!! You’ve got the best Dads in the world to celebrate with!! Merry Christmas," another enthused.
Others pointed out the tennis player on the cake, which offered a rare insight into the teen's sporting interests.
Elton and David - who tied the knot in 2014, nine years after their civil partnership ceremony in 2005 - are also parents to a 10-year-old son named Elijah.
The couple opted for surrogacy via the Center for Surrogate Parenting in Encino, California.
According to the MailOnline, the couple reportedly paid the surrogate, whose identity has never been revealed, a lump sum for giving birth to their firstborn.
Three years later, the couple used the same surrogate to welcome their second son.
Christmas is taken extremely seriously in Elton's house. In past years, the family dressed up as elves to dance to the songwriter's iconic Christmas hit, Step into Christmas, which you can see in the video below.
The star previously revealed to Good Housekeeping what a special time of year it is, making memories and creating new family traditions with their boys.
He told the publication: "When it was just me and David, it used to be low-key and grown-up: we’d watch a film on Christmas Eve and have friends over for lunch the next day, or go to Venice, which is wonderful at that time of year.
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"Now, it's all about Zachary and his little brother Elijah, although our friend Sandy often visits; she and her late partner Ingrid were the boys' godparents. It's about making your own family traditions, some of which are the same as those I remember as a kid: turkey with all the trimmings, cold cuts and Branston pickle on Boxing Day, paper hats and balloons."