The Steam Deck will go on sale on February 25th. Customers who have reservations will get an email on that day and have three days to place an order. If you have a reservation, March will be the time to keep an eye on your email because Valve will release new batches on a weekly basis.
This is some more information from Valve's announcement.
We will start sending invites shortly after 10:00 am on February 25th, PST
Order emails are sent in the same order that reservations were made.
You can only order the Steam Deck model that you originally reserved.
Your reservation deposit will be applied to the final price of Steam Deck, and shipping costs are included.
The release date is in line with Valve's prediction that the console would be out by the end of February. On the 28th, Valve says that the orders will start shipping out.
It is good to have a date for the handheld console. The Steam Deck was supposed to ship in December of that year. The launch was delayed due to the supply chain and shortage issues that have become standard with many product launches.
If you haven't already, you can reserve the steam deck for $5. The order page says you can expect to order after Q2 2022. if you're still on the fence.