Rashi on Leviticus 23:15/רש"י על ויקרא כ״ג:ט״ו

1ממחרת השבת FROM THE MORROW AFTER THE DAY OF REST — i. e. from the morrow after the first day of the Passover festival (Menachot 65b).

2תמימת תהיינה [SEVEN WEEKS] SHALL BE COMPLETE — This teaches that one has to begin counting in the evening, for otherwise they would not be complete (Menachot 66a).