In the first month (rishon), on the fourteenth day of the month at evening, you shall eat unleavened bread. . . (Ex. 12:18)
The School of R. Ishmael taught: We find that the fourteenth is called the “first”, as it is said, on the first, on the fourteenth day of the month.
R. Nahman b. Isaac said: ‘The first’ [ba-rishon] means the preceding, for the Scripture says: Wast thou born, before [rishon] Adam? (Job 15:7 – lit. “Are you the first man born?”)
If so, [what about], and ye shall take you out the first [rishon] day, [the boughs of goodly trees. . . ](Lev. 23:40) — does ‘rishon’ here too mean the preceding? — There it is different, because it is written, and ye shall rejoice before the Lord your God seven days: (ibid.) Just as the seventh [means] the seventh of the Festival, so the first [means] the first of the Festival. . .
For the School of R. Ishmael taught: As a reward for [the observance of] the three ‘firsts’ [Israel] merited three firsts. . .:
To cut off the seed of Esau - of whom it is written, And the first came forth red, all over like an hairy garment; (Gen. 25:25)
The building of the Temple - whereof it is written, A glorious throne, set on high from the first is the place of our sanctuary; (Jer. 17:12)
And the name of Messiah - for it is written, First unto Zion, behold, behold them. (Is. 41:27)