referring to the silvery moon, or the moon shining on the water and making it look silvery

refers to the controversy in early church as to whether Easter should be on a Sunday or on the 14th day . See here

http://en.wikipedia.org/wiki/Quartodecimanism

Very early in the life of the Church, disputes arose as to which date Pasch or Easter (called "Pascha" in Greek and Latin) should be celebrated. Disputes of this kind came to be known as Paschal/Easter controversies. The first recorded such controversy came to be known as the Quartodeciman controversy.

In the early period, Easter was always held on a date near the middle of the Jewish month of Nisan. In the mid–second century A.D., the practice in the Roman province of Asia was for the pre-Easter fast to end on the eve of the 14th day of Nisan, regulated by the full moon, the day on which the Passover sacrifice had been made when the Second Temple stood, and "the day when the people put away the leaven"[2] (such as Jews, Jewish proselytes, and Jewish Christians). Nisan 14 itself was commonly, if somewhat confusingly, also called Passover; technically it is Preparation Day for the seven-day Feast of Unleavened Bread that begins on Nisan 15, now also called Passover. The Asian custom became known as "Quartodecimanism" among the Latins. Melito of Sardis (d. c.180) was a notable Quartodeciman.

The practice elsewhere was to continue the fast until the eve of the Sunday following; the objection to the Quartodeciman practice was that the 14th of Nisan could fall on any day of the week. Outside of Roman Asia, Christians wished to associate Easter with Sunday, the day on which Jesus rose from the dead according to all the Gospels, and which had long been a Christian holy day,[3] known as the Lord's Day. According to the writings of Irenaeus (d. c. 202), the Roman church had celebrated Easter on a Sunday at least since the time of Bishop Xystus (Sixtus I, 115–125).[4]


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Note added at 38 mins (2012-04-25 22:41:24 GMT)
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need more context to see how this works into your translation

the main problem of course was (as stated above) that the 14th could fall on any day of the week. The Council of Nicosia was convened to deal with this in 325.

NB
"Jehovah's Witnesses continue to celebrate the memorial of Christ's death on Nisan 14" ...(from wiki)

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Note added at 1 hr (2012-04-25 23:27:48 GMT)
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need more context for the "plateado" as could be moonlight in Garden of Gethsemene or the 30 pieces of silver given to Judas. Thurs =the 14th since the 10th is "domingo" and of course the moon still determines the date of Easter (and Passover)

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Note added at 11 hrs (2012-04-26 09:19:55 GMT)
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I see now from your further question that this is to do with Judaism rather than Christianity. So the previous comments about the 14th Nisan apply. The "Plateado" referring to the moonlight of the full moon that precedes the 14th. SO,

THE MOONLIT THURSDAY, 14TH OF NISAN