Midrash su Genesi 1:15

And God said "Let there be luminaries" (Genesis 1:14) R. Yohanan expounded "He made the moon for the holidays:" (Psalms 104:19) Said R. Yohanan: "[The moon] was not created to cast light; only the sun was. Why then was the moon created? In order to sanctify by its reckoning the Firsts of the Months and years." R. Shiloh of Kh'far T'marta said in the name of R Yohanan, "Even though 'he made the moon for the the holidays,' 'the sun knows when to set,' (Psalms 104:19) meaning, by the sun is its setting known, meaning that we only count by the moon once the sun sets. Yusti Havra said in the name of R. Berekhyah, "Behold it says 'And they departed Ra'meses in the first month on the 15th of the month,' and if you count according to the moon, it was still only the 14th -- behold we do not count according to the moon but rather from the setting of the sun." R. Azaryah said in the name of R. Hanina, "[The moon] was not created to cast light; only the sun was. Why then was the moon created? Rather, it teaches that the holy, blessed He foresaw that in the future the idolaters would make [the luminaries] into gods. The holy, blessed He said, 'If the idolaters make two competing [luminaries] into gods, all the more so would they make one [luminary into a god].'" R. Berekhyah said in the name of R. Simon, "Both were created to cast light, for it says 'And they will serve as luminaries to shine upon the earth,' (Gen. 1:15) and it says 'And God gave them into the firmament of the heavens to shine upon the earth.' (Gen. 1:17) [The Midrash continues to expound Gen. 1:14.] And they will be for signs: these are the sabbaths. And for holidays: these are the Three Pilgrimages. And for the days: these are the Firsts of the Months. And for the years: this is the sanctification of the years."