1. It is positive commandment to count seven complete weeks from the day that the “Omer of the waving” is offered, as it is written: You shall count for yourselves—from the morrow of the rest day—seven weeks (Vayikra 23:15). The mitzvah is to count the individual days along with the weeks (Rambam, Temidin Umusafin 7:22). However, most authorities maintain that nowadays, when there is no Omer offering, the mitzvah of counting is Rabbinical ordained, [not Scripturally mandated as it was in the time of the Temple].
2. Sefiras Ha’omer (“the Counting of the Omer”) is counted every evening, starting from the conclusion of the first day of Pesach until the festival of Shavuos. Ideally, one should count at the beginning of the night. But if one forgot to do so, he may recite the blessing and count any time during the night. If the person did not count at all during the night, he should count during the following] day but without reciting the blessing. The person may then continue counting with the blessing on the subsequent nights (Shulchan Aruch OC 489).
