The Gaon Rabbi Yehuda Aryeh Diner, the central rabbi of Bnei Brak, instructs those asking that this year Ashkenazim are permitted to get their hair cut on the eve of Rosh Chodesh Iyar - because Rosh Chodesh falls on Shabbat.
According to Jewish law, from the end of the first day of Passover until Lag BaOmer, mourning customs are observed, including the prohibition of cutting hair and shaving - and many observe this practice until the eve of Shavuot.
The Shulchan Aruch states that "some people have the custom of cutting their hair on Rosh Chodesh Iyar, and it is their mistake." The Rema's words also seem to agree with the Shulchan Aruch's view that Rosh Chodesh Iyar does not have a special status, but he notes that some people have the custom of not observing the mourning customs in the month of Nissan itself and only begin on the second day of Rosh Chodesh Iyar. This is how the Chayei Adam rules.
However, this year there is a special permission, as stated by the Posk, the Gaon Rabbi Yehuda Aryeh Diner - that it is permissible to get a haircut on Friday, Rosh Chodesh Iyar.
Thus it is written in the issue of 'Mim Chaim' from its halakhic code: "Even for those who practice mourning customs before Rosh Chodesh Iyar, especially this year (583) when Rosh Chodesh fell on Shabbat, since there is a Shabbat celebration and Rosh Chodesh, it is permissible to have their hair cut on Shabbat eve out of respect for Shabbat, (Mishnab 603, 505). However, those who practice all year long, in accordance with the will of Rabbi Yehuda the Hasid, not to have their hair cut on Rosh Chodesh, are also forbidden from this (Garish Elyashiv zt"l and Baal Shabbat Halevi zt"l, which the Mishnab did not mention here because it relied on what is already stated in the halakhic code of Shabbat, 603, 7), but they can have their hair cut on Thursday, the eve of Rosh Chodesh Iyar.
""And we mean: since Rosh Chodesh Iyar falls on Shabbat, there is a special permit to have a haircut in honor of this Shabbat, which has both Simchat Shabbat and Rosh Chodesh, and whoever is careful, as the will of the Chasid, not to have a haircut on Rosh Chodesh, can have a haircut in honor of Shabbat on Thursday, the eve of Rosh Chodesh Iyar.".
