The official launch day means that Valve will start sending emails to the first batch of people who made a reservation for the Steam Deck device. If you’re reading this and you are one of the first to have made a reservation, you have 72 hours, from the moment you receive the email, to complete your purchase on Steam.
If you changed your mind and no longer want to buy the Steam Deck, or you miss the 72-hours window and don’t purchase the device, Valve will send your reservation to the next person in the queue. Meanwhile, Valve is working to send out the official Dock for Steam Deck to customers too.