"This is an incredibly difficult week for Sierra's family," said Marc Klaas, head of the KlaasKids Foundation.
Sierra disappeared on her way to her high school in Morgan Hill on the morning of March 16.
Since her disappearance, the LaMar family has teamed up with KlaasKids to organize searches for Sierra.
This week also marked seven months since Sierra's disappearance -- another so-called "milestone" that Klaas said "are reminders of what you can't forget."
Klaas commiserated with Sierra's parents, Marlene and Steve, that occasions such as birthdays can be difficult.
"They would so much rather be celebrating by singing 'Happy Birthday' to her," he said.
Klaas' 12-year-old daughter Polly was kidnapped from her mother's Petaluma home and was missing for two months before she was found murdered in December 1993.
On social media, Facebook posts and Twitter updates poured in today wishing Sierra LaMar a happy birthday and her safe return to her family.
A weekly search from the Find Sierra LaMar Search Center at Burnett Elementary School at 85 Tilton Ave. is scheduled starting at 8 a.m. Saturday.
"Searches are going to continue on Saturdays as long as the family wants them," Klaas said.
Last month, searches were pared down to only Saturdays instead of previous bi-weekly search efforts on Wednesdays and on the weekend.
Klaas said numbers "erode over time" but a core group of volunteers comes out each Saturday, combing through the Santa Clara County area looking for Sierra.
Amid the search for Sierra, Antolin Garcia-Torres, 21, has been charged with her kidnapping and murder after DNA evidence believed to be Sierra's was found in his car.
He was arrested on May 21 but has yet to enter a plea.