120:10 It is the custom for both men and women to refrain from work during the Omer period from sunset until after counting. A hint to this is the phrase “seven weeks” (Leviticus 21:15); the word “shabasos” (weeks) is related to “shvus” (rest). We infer that from the time of the counting, which begins at sunset, one should refrain from labor until after he has counted.
120:11 On the first night of Shavuos, we delay reciting the evening prayer service until after the stars come out. This is because if we prayed sooner and accepted yom tov early, we would not have 49 complete days of counting. The Torah tells us (Leviticus 23:15) that there must be seven complete weeks.