Friendships cross boundaries of religion, culture, age, income level, and education. Besides that, they’re also key to our personal happiness and emotional well being. So, it may come as a surprise that National Friendship Day wasn’t invented until the early 20th century — and only then as a way to increase sales of greeting cards. But when the greeting card industry declined mid-century, National Friendship Day remained strong. Now this holiday is more popular than ever — thanks mostly to social media. On the first Sunday in August, we celebrate our friendships by saying “Thank you for being a friend.”