Le Deutéronome 16:9 Halakha : Mishneh Torah, Prayer and the Priestly Blessing & Sefer HaChinukh

On Pentecost, the section from the Pentateuch is that beginning, "Seven weeks" (Deuteronomy 16:9-17). But the general custom is to read on the first day of the feast the section beginning, "In the third month" (Exodus 19:1-20:26). And the lesson from the Prophets is "The vision of the Chariots" (Ezekiel 1:1-28). On the second day the section read is that concerning the Festivals, beginning, "All the firstling males" (Deuteronomy 15:19-16:17) and the lesson from the Prophets is taken from Habakuk (3:1-end).

The commandment of counting the omer: To count forty-nine days from the bringing of the omer which is on the sixteenth day of Nissan, as it is stated (Leviticus 23:15), “And you shall count for yourselves from the morrow of the Shabbat from the day of your bringing the omer of waving.” And this tallying is an obligation, and it is upon us to count the days on each day and, likewise, the weeks. As Scripture said to count fifty days and also said (Deuteronomy 16:9), “Seven weeks shall you count for yourself.” And in explanation, Abbaye said in the Gemara in Menachot 66a, “It is a commandment to tally the days and it is a commandment to tally the weeks.” (See Sefer HaMitzvot, Positive Commandments 161.) And there are some of the commentators (see the end of Ran on Pesachim) who opine that the intention of the verse is to tally the weeks specifically when they are full, but there is no need to mention [them] every day and say that they are such-and-such days and such-and-such weeks. And there are some that say (Rosh in his Responsa 24:13) that the [proper] way is to mention the tally of weeks with the days always on every day. And one who fears the Heavens will choose their way to remove [himself] from any doubt, and not be concerned about the elegance of the words. And so have they practiced today in all places of which we have heard.