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Friends pose for a selfie following a naturalization ceremony at Oakton High School on Dec. 22 in Vienna, Virginia. Getty Images

Sunday is Friendship Day, a United Nations-approved celebration of the relationships between people, countries and cultures. Typically celebrated the first Sunday of August, Friendship Day was initially created in 1919 as a Hallmark Holiday, according to daysoftheyear.com. These days, it's simply an excuse to hang out with your BFFs.

We all know friends make the world go 'round, so why not observe Friendship Day by letting your confidants know how much you appreciate them? In order to help you do that, here are 17 quotes about friendship, as collected from BrainyQuote, Goodreads and Thought Catalog. Share them and smile.

"Rare as is true love, true friendship is rarer." — Jean de La Fontaine

"You only really fall apart in front of the people you know can piece you back together." — Sarah Dessen

"A friend is someone who knows all about you and still loves you." — Elbert Hubbard

"There is nothing I would not do for those who are really my friends." — Jane Austen

"Friendship is mutual blackmail elevated to the level of love." — Robin Morgan

"Friendship... is not something you learn in school. But if you haven't learned the meaning of friendship, you really haven't learned anything." — Muhammad Ali

"Some people go to priests, others to poetry, I to my friends." — Virginia Woolf

"Love is blind; friendship closes its eyes." — Friedrich Nietzsche

"Most of us don't need a psychiatric therapist as much as a friend to be silly with." — Robert Brault

"Friendship is born at that moment when one person says to another: 'What! You too? I thought I was the only one.'" — C.S. Lewis

"Be slow in choosing a friend, slower in changing." — Benjamin Franklin

"A friend is someone who gives you total freedom to be yourself." — Jim Morrison

"She is a friend of mind. She gather me, man. The pieces I am, she gather them and give them back to me in all the right order. It's good, you know, when you got a woman who is a friend of your mind." — Toni Morrison

"What is a friend? A single soul dwelling in two bodies." — Aristotle

"There's no way for them to take away my sadness, but they can make sure I am not empty of all the other feelings." — David Levithan

"Be slow to fall into friendship, but when you are in, continue firm and constant." — Socrates

"My friends have made the story of my life. In a thousand ways they have turned my limitations into beautiful privileges." — Helen Keller