The superstition: “It’s bad luck to celebrate a person’s birthday before it happens. It’s because people can’t possibly know that they will make it to your birthday, so to celebrate beforehand is the opposite of humble, I can’t think of the word right now.”
The informant is Indian American. Her parents are both from India, but she was born in California. She’s not very religious, but she considers herself culturally Indian. She grew up hearing this superstition from her parents, so she has always followed it. It seems rooted in spirituality, if not outright religion, which matches the informant’s cultural sense of being Indian without being religious. The reason for the superstition makes sense to me, that you’re never sure of the next day, so don’t be presumptuous when thinking about the future–to live every day grateful for simply waking up. It also mirrors beliefs outside the Indian culture, such as Christian prayers thanking God.