Ibn Ezra on Leviticus 23:23

וטעם להזכיר וידבר ה׳‎ עם יום התרועה בעבור היותו מועד בפני עצמו כי חג שבועות תלוי מיום תנופת העומר וכן הזכיר עם הצום גדול ועם חג הסוכות: [SPOKE.] The reason the statement And the Lord spoke precedes the law dealing with the day of the sounding of the shofar¹⁸³Which it does not do with regard to the festival of Shavu’ot. is that the latter is a festival unto itself.¹⁸⁴The laws dealing with the festival of Rosh Ha-Shanah opens with And the Lord spoke unto Moses saying. The preceding festival of Shavu’ot does not have this introduction. Hence I.E.’s comments. However, the festival of Shavu’ot is connected to the day the omer is waved.¹⁸⁵Hence it does not open with And the Lord spoke unto Moses saying. And the Lord spoke is similarly written in connection with the great fast¹⁸⁶Yom Kippur. and the festival of Sukkot.