Shalom. The Mechaber in Orach Chaim 493:2 says that one should not trim his hair until the 33rd day of the Omer and in the same sentence (without saying Yesh Omrim) says to wait until the morning of the 34th day. Why does the Mechaber quote two times? When does the ‘morning’ (of the 34th day) begin in this context (Amud HaShachar, Zericha…)? Thank you.
The Mishna Berurah explains the Mechaber, that he means not to take haircuts until (meaning until) after 33 full days, however we should still wait until the morning of the 34th day, since they were still died on the 34th day. However since it was much less, it is only for half the day. The minhag of the Ashkenazim is to take haircuts from the morning of the 33rd day (in accordance with the opinions that they stopped to die on the 33rd day).
Daytime is usually from Alos or Amud Hashachar. As a side point this question is only theoretical, because many poskim say that we should not take haircuts before davening shacharis
O:CH 90-7, M:B 36, Halichos Shlomo Teffilah 2-7, Ohr L’tzion vol. 7-8.