Kate Middleton Delivers Holiday Message, Mentions Times of Sadness in Post

Kate Middleton is spreading positive holiday cheer with an uplifting message, while also mentioning "times of joy and sadness" ahead of her annual carol concert.

The Princess of Wales -- who is continuing her recovery from cancer -- shared a heartfelt clip to her social media Tuesday ... urging everyone to “shine for each other.”

The clip shows various images from the Christmas Carol event Middleton puts on each year, which was recorded at Westminster Abbey earlier this month ... with kids hanging decorations, playing steel drum and -- of course -- singing their hearts out.

In the clip, Princess Kate talks about all the inspiring people in the United Kingdom ... embracing those in need -- saying this is the time of year when all people must shine for each other.

She then mentions that "in times of joy and sadness, we are all each other's light." Middleton ends her message by wishing everyone and their families a Merry Christmas.

Middleton and her family have certainly felt both joy and sadness this year ... 'cause while the princess is attending more public events, she's still fighting off cancer.

As you know ... Kate announced her diagnosis in a video message back in March -- after speculation ran wild regarding her whereabouts.

Throughout the rest of the year -- which her husband, Prince William, called the probably the most difficult of his life -- Kate has slowly returned to public duties ... making an appearance at Trooping the Colour in June and Wimbledon in July.