The Torah (Leviticus 23:15-16) requires one to count both the days and the weeks of the omer. On the first day one counts “Yom echad la’omer” not “Yom rishon la’omer” so as to make it perfectly clear that the first day of the omer is the day following the first day of Pesach and not the day following Shabbat.
In so doing we reject the opinion of the Sadducees who interpreted the words “mimochorat hashabbat” (Leviticus 23:16) to mean Sunday. Because the Torah requires one to count “seven complete weeks” one should count at the beginning of the day, which in Jewish law begins on the preceding night. The optimum time to recite Sefirat HaOmer is after Maariv immediately following the appearance of three stars. If however it is difficult to find a minyan at that late hour, one should recite Sefirat HaOmer with an earlier Maariv minyan, even at plug haMinchah time, without a blessing, rather than wait and count alone.
On Friday night and Yom Tov night, Sefirat HaOmer is recited in the synagogue after Kiddush and at home before Kiddush. On Motzaei Shabbat, Sefirat HaOmer is recited after Kaddish Titkabal and before Havdalah. Based on the verse referring to “standing grain” (Deuteronomy 16:9), one should count standing. As with the lulav, so too with the omer, one must count oneself (ulekachtem lachem, usefartem lachem) and one cannot rely on someone else to count on one’s behalf. Women are not obliged to count but may volunteer to count the omer with a blessing.
The verse that requires one to count “seven complete weeks” means that if one missed a whole day and did not remember to count until the following night, one can no longer recite the blessing before counting on the following days. If, however, one remembered one’s omission during the day, including at twilight, before nightfall, one may count that day without the blessing, and on the following days one may count with a blessing. Based on the majority opinion that Sefirat HaOmer today is of rabbinical origin, if one is not sure whether one counted the previous day, one may continue to count the following days with a blessing. If one is asked at twilight what day of the omer it will be tonight, one should reply with yesterday’s count, but if asked before twilight one may reply with tonight’s count.
According to some opinions, a rabbi or a chazzan who leads the counting of the omer may recite the blessing, even if he knows he omitted a previous day’s count. A person who crosses the dateline and gains or loses a day may, according to certain halachic opinions, disregard the local count and continue counting, as he would back home.