Midrasch zu Wajikra 23:5

1) (Vayikra 23:5) ("In the first month, on the fourteenth day of the month, at twilight, it is Pesach to the L–rd.") I might think, when it gets dark; it is, therefore, written "day." If "day," I might think from the second hour (of the day); it is, therefore, written "at twilight." Just as twilight marks the "turning" of the day (towards evening), so, "day" (here) marks the turning of the day, from the sixth hour on. And even though there is no proof for this, it is intimated (in Yirmiyahu 6:4) "Woe unto us for the day is turning, for the shadows of evening have stretched forth."

6) I might think that just as the year is intercalated to answer some special need, so, it is intercalated because of tumah (of the populace); it is, therefore, written (Vayikra 23:5) "In the first month … it is Pesach" — Let the first month not pass without Pesach.