Tertullian, On Baptism 19
Easter outranks Passover as the day for baptism, because the Lord's passion -- the very death we are plunged into -- has been accomplished, and the typology fits without strain: when Jesus was about to eat his final Passover, he sent the disciples ahead with the sign, "You will find a man carrying a water jar," and that water pointed to the place where the feast would be kept (Mark 14:13-15; Luke 22:10-12).
From that moment Pentecost opens a broad field for scheduling baptisms, since the risen Lord appeared repeatedly to his disciples then, the Holy Spirit's grace was poured out, and the hope of his return was signaled when the angels told the apostles, "He will come back in the same way you saw him go into heaven" (Acts 1:11) -- a promise stamped with Pentecost's own date.
Jeremiah clinches the double reference: "I will gather them from the ends of the earth on a feast day" (Jeremiah 31:8), naming both Passover and Pentecost, the latter being the feast day proper.
Even so, every day belongs to the Lord, every hour, every moment is fit for baptism; if the only difference is solemnity, grace itself makes no distinction.
Latin
Diem Baptismo solemniorem Pascha praestat, cum et passio Domini in qua tinguimur, adimpleta est, nec incongruenter ad figuram interpretabitur, quod cum ultimum Pascha Dominus esset acturus, missis discipulis ad praeparandum, Invenietis, inquit, hominem aquam bajulantem Paschae celebrandae locum de signo aquae ostendit. Exinde Pentecoste ordinandis lavacris latissimum spatium est, quo et Domini resurrectio inter discipulos frequentata est, et gratia Spiritus sancti dedicata, et spes adventus Domini subostensa, quod tunc in coelos recuperato eo, Angeli ad apostolos dixerunt, sic venturum quemadmodum et in coelos conscendit utique in Pentecoste. Sed enim Jeremias cum dicit: Et congregabo illos ab extremis terrae in die festo, Paschae diem significat et Pentecostes, qui est proprie dies festus. Caeterum omnis dies Domini est, omnis hora, omne tempus habile Baptismo: si de solemnitate interest, de gratia nihil refert.