The prohibition to cut hair during the Moed:[1]

It is forbidden to cut one’s hair during the Moed.[2] This applies even if one cut his hair prior to the Moed.[3] This applies even if one is doing so in order to beautify himself for the sake of the Moed.[4]

[1] Michaber 534:2; based on Mishneh Moed Katan 13b

[2] The reason: The Sages prohibited cutting hair during the Moed in order so one cut his hair before Yom Tov and not enter the Moed looking unbefitting. [M”A 531:1; Taz 531:1; M”B 534:3]

[3] Michaber ibid; Tur; Hagahos Ashriy; Or Zarua

The reason: As people will not know that he got a haircut before the Moed and they will think that he is transgressing and cutting his hair initially during the Moed. [Taz 531:2; M”B 531:3]

[4] M”B 531:3