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From BoLT1; "The Theft of Melko” :

“….they made merry on one day every seventh year to celebrate the coming of the Eldar into Valinor, and every third year a lesser feast to commemorate the coming of the white fleet of the Solosimpi to the shores of Eldamar; but at every twenty-first year when both these feasts fell together they held one of the greatest magnificence, and it endured for seven days, and for this cause such years were called "Years of Double Mirth"; and these feasts all the Koreldar wherever they now may be in the wide world still do celebrate…”

“…..Now that feast that approacheth is one of Double Mirth, and all the hosts of the Gods and Elves made ready to celebrate it most gloriously. Pomps there were and long processions of the Elves, dancing and singing, that wound from Kôr to Valmar's gates. A road had been laid against this festival from the westward gate of Kôr even to the turrets of the mighty arch which opened in the walls of Valmar northward towards the Trees….”

“….Now their custom was on the third day to robe themselves all in white and blue and ascend to the heights of Taniquetil, and there would Manwë speak to them as he thought fit of the Music of the Ainur and the glory of Ilúvatar, and of things to be and that had been. And on that day would Kôr and Valmar be silent and still, but the roof of the world and the slope of Taniquetil shine with the gleaming raiment of the Gods and Elves, and all the mountains echo with their speech -- but afterward on the last day of merriment the Gods would come to Kôr and sit upon the slopes of its bright hill, gazing in love upon that slender town, and thereafter blessing it in the name of Ilúvatar would depart ere Silpion came to bloom; and so would end the days of Double Mirth.”