When, just after, as in Please call me as soon as dinner is ready, or As soon as the sun goes down, the temperature drops dramatically. [Late 1200s]
At the earliest moment that, as in Telephone me as soon as you can. It often takes the form as soon as possible, meaning at the earliest possible moment, as in He’ll finish the work as soon as possible. This expression employs possible in the sense of “if it can or could be,” a usage dating from the late 1600s.