19 That mention is made of the twenty seventh day of the month; as we have already seen the significance of the square in the beams. Here especially it is significant; for as twenty-seven is the cube of three, there is a trinity in the means by which we are, as it were, squared, or fitted for every good work. By the memory we remember God; by the understanding we know Him; by the will we love Him. That in the seventh month the ark rested; reminding us again of the seventh day of rest.
And here again, to denote the perfection of those at rest, the twenty-seventh day of the month is mentioned for the second time. So what is promised in hope is realized in experience. There is here a combination of seven and eight; for the water rose fifteen cubits above the mountains, pointing to a profound mystery in baptism—the sacrament of our regeneration. For the seventh day of rest is connected with the eighth of resurrection. For when the saints receive again their bodies after the rest of the intermediate state, the rest will not cease; but rather the whole man, body and soul united, renewed in the immortal health, will attain to the realization of his hope in the enjoyment of eternal life.
Thus the sacrament of baptism, like the waters of Noah, rises above all the wisdom of the proud. Seven and eight are also combined in the number of one hundred and fifty, made up of seventy and eighty, which was the number of days during which the water prevailed, pointing out the deep import of baptism in consecrating the new man to hold the faith of rest and resurrection.
Source: Reply to Faustus the Manichaean (New Advent)