Exodus 12:3

In the tenth day. Though the lamb designated for the Passover feast was not to be slain and eaten before the 14th day of the month (v. 6), preparations for the feast were to be started 4 days in advance.

A lamb. The Hebrew word is applicable to both sheep and goats, without limit as to age. However, by enactment the age was fixed (see v. 5) at one year, and a man was free to select either a lamb or a goat (v. 5). It is interesting that the Hebrews in general preferred a lamb to a kid, and with one recorded exception (2 Chron. 35:7), seem never to have used anything else for the Passover ritual.