Do I need to buy tickets in advance to go to Luang Prabang?

Generally, it is fine to get the long-distance bus to Luang Prabang on the spot, at KM Southern Bus Station. But there is only one bus in operation every day, to ensure your trip, you can book as well. The reservation is only accessible around 5 days before. And if necessary, here is the number for bus ticket reservation:
400-651-0871
Hope help!

Thanks! :)