Ramban

SPEAK UNTO THE CHILDREN OF ISRAEL, AND SAY UNTO THEM: WHEN YE ARE COME INTO THE LAND. Since He establishes a new commandment here in connection with each of the festivals, in addition to their being days of rest and holy convocation, therefore He singled out for each festival a special section, which begins with the charge, Speak unto the children of Israel. And since the time of the Festival of Weeks is dependent upon the omer,,” and on the fiftieth day is the Festival of Weeks. Thus it is clear that the time of this festival is dependent upon the omer. therefore He stated it all in one section. And because the Day of Remembrance [i.e., the New Year] and the Day of Atonement are in one month [Tishri], and both relate to one subject — the [Divine] Judgment concerning man’s sins and the atonement [achieved] by those who do repentance — therefore He did not say concerning the Day of Atonement “speak unto the children of Israel,” since the first statement [about the New Year] was sufficient in this case. He made, however, two separate sections [of the omer and the Festival of Weeks, as also the New Year and the Day of Atonement], because they are different matters.
AND YE SHALL REAP THE HARVEST THEREOF. This means that you are not to reap any harvest in the Land until you have reaped the omer,” and on the fiftieth day is the Festival of Weeks. Thus it is clear that the time of this festival is dependent upon the omer. and have brought it as the first-fruits of your harvest unto the priest. Similarly, from the time the sickle is first put to the standing grain shalt thou begin to number seven weeks, means “from the time you begin to put the sickle to any standing grain, for it is forbidden to raise the sickle over standing grain until the day that you begin to count [seven weeks].”