On New Year’s Eve, millions of people will use Facebook’s Messenger app to wish friends and family a “Happy New Year!” If everything goes smoothly, those messages will reach recipients in fewer than 100 milliseconds, and life will go on. But if the service stalls or fails, a small team of software engineers based in the company’s New York City office will have to answer for it.

Isaac Ahdout, engineering manager, and Thomas Georgiou, software engineer, are both on that team. They’ve tested and tweaked the app throughout the year and will soon face their biggest annual performance exam. Messenger’s 1.3 billion monthly active users send more messages on New Year’s Eve than on any other day of the year. Many hit “send” (represented as a blue arrow in the app) immediately after the clock strikes midnight in their respective time zones.

“There’s like this firehouse you can’t stop, of deliveries you have to make,” says Georgiou. “We have to keep up. Otherwise, you end up in a bad situation.”

It’s a problem familiar to anyone who works on networks or services that see a dramatic spike in use at a particular time of day or year. U.S. telecommunications companies frequently install new base stations ahead of Super Bowls, state fairs, and presidential inaugurations for similar reasons.

For Facebook’s Messenger team, the challenge is slightly more complicated than shuffling a simple message from one user to another. Facebook allows people to set up large group chats, and shows senders a receipt every time a message is delivered, sent, or read. These features compound the total number of messages that must be distributed across the service.

Users also send and receive a higher percentage of photos and videos as they ring in the new year, compared to an average day. And people often try to resend messages that don’t appear to make it through right away, which piles on more requests.