Sefirat Haomer: A Framework For a Dynamic Lifestyle

And from the day on which you bring the sheaf of elevation offering—the day after the sabbath—you shall count off seven weeks. They must be complete: you must count until the day after the seventh week—fifty days; then you shall bring an offering of new grain to the LORD.

It is a biblical positive command to count seven full weeks from the day that the omer is brought, as it is written: "From the day after the sabbath, the day you bring the sheaf of wave-offering, you shall count seven full weeks" (15). The days must be counted along with the weeks, as it is written: "You shall count fifty days" (16). The counting must be performed from the beginning of the day. Accordingly, one is to do the counting at night, beginning with the night preceding the sixteenth of Nisan. If one forgot and failed to count at night, he may count at daytime. The counting must be performed while standing; but if one counted while sitting, he has fulfilled his duty. Every Jew is required to perform this duty, everywhere and at all times. Women and slaves, however, are exempt from it.

The Gemara asks: And now that we know the determination of the first day of the new month, what is the reason that we observe two Festival days in the Diaspora? Because they sent a warning from there, from Eretz Yisrael: Although now there is a fixed calendar and there is no uncertainty, be careful to observe the custom of your fathers that you received, because at times the monarchy will issue decrees of persecution restricting Torah study and the fixed calendar may be forgotten. And the people will come to have their proper observance of the Festivals be disrupted again. However, the fundamental halakha is that the observance of two Festival days is based on uncertainty.