To mark the date, supporters will gather in small and large groups across the country.
Sarah Shourd's brother and mother will take part in the Berkeley vigil at 8 p.m. on Ellis Street.
Chris Rapp, Shourd's brother, flew in from Boise Idaho. "It's a bit difficult for me. I'm the older brother. I'm supposed to make sure she's safe and stuff. And when you can't do anything about that, it's kinda tough," Rapp said.
Shourd's mother met with Secretary of State Hillary Clinton in the past week in what she describes as an emotional meeting. Clinton met with the mothers of all three hikers.
"We talked a lot like mother to mother with her which felt very good to all of us mothers. It's important for us for people to understand how important our children are to us," Nora Shourd said.
She said Secretary Clinton assured them that the U.S. government is doing everything it can to secure the hikers' release on humanitarian grounds.
The United States and Iran don't have diplomatic relations, so a Swiss diplomat is the only person allowed to visit the three. So far there have been two visits and the three were described as in good physical condition.
"Psychologically this must be very hard for them and we just hope that the Iranian officials will find an opportune moment to let them go," Shourd said.
The family also referred to recently released video taken by a fourth hiker that shows the three having fun on their vacation. They say it shows they posed no threat to anyone.
The family says waiting is the hardest part. "It could be over tomorrow. It could take a while. That's the hardest part. There's no end to the project. We don't know when it's going to end," Rapp said.
Also Sunday the family released the following statement:
One hundred days ago today, Shane, Sarah and Josh were detained in Iran. 100 days of silence, 100 days of absence, 100 days too long. Their continued detention is unjustified and we call on the Iranian authorities to release them without delay.
As long as Shane, Sarah and Josh remain in detention, our efforts to win their freedom will continue. We ask everyone here, and everyone who has shown their support around the world, not to let up until they are back in our arms.
We know that Shane, Sarah and Josh feel your presence here today and that it brings light and love to the dark loneliness of their cells in Evin Prison.
Last week, Shane, Sarah and Josh sent private messages to their families through the Swiss diplomats who were able to visit them.
Shane asked his family to give you this message: "For the people who gathered at the vigils, I want them to know that we can feel it."
And this is what Sarah wanted everyone to hear: I know you are fighting for me and it makes me proud. I am hanging in there with you.
One way you can support Shane, Sarah and Josh is to send them brief messages of solidarity. We are asking everyone to send messages on a postcard at this vigil. We know previous letters they have received have helped to ease their isolation and we will make sure your messages reach them.
Josh wrote his family: Thanks for all the wonderful letters. I spend a lot of time reading [them].
Thank you again for everything you are doing to try to secure the release of Shane, Sarah and Josh. It comforts them in their time of need.