The Magen Avraham writes, that when Tisha B’Av falls on Thursday: הכל מותר לכבוד שבת – “everything is permitted in honor of Shabbos”. The Hagaos Boruch Ta’am (558) asks, that there is no prohibition to have a haircut after midday on Friday, so why should there be a heter to have one any earlier than that? The Eshel Avraham also seems to concur with the Hagaos Boruch Ta’am and he adds, that we don’t need to be concerned about people missing Mincha, as there is a shamash [attendant] who goes around the city calling and reminding people to daven Mincha (see Mishnah Berurah 251:7). Nowadays, we no longer have such a shamash, therefore, perhaps we can no longer make such an argument, however, in Eretz Yisroel and in many other places there is a siren that is sounded to warn people that Shabbos is soon arriving, so why should there be any heter to have a haircut before midday on Friday?
In spite of the above, the custom seems to be that although people refrain from having a haircut on motzei Tisha B’Av, from Friday morning and on people are lenient. However, if one has no choice and the only time he is able to have a haircut is on motzei Tisha B’Av, then as long as it is being done in honor of Shabbos, just like many poskim are lenient with regards to washing clothes, similarly, many poskim are lenient in regards to having a haircut.
