[The Ruling on One Who Entered Iḥrām for ʿUmrah at the End of the Days of Tashrīq out of Ignorance]

Answer—and Allah grants success: He acted improperly thereby; however, the legally responsible person may be excused for ignorance and mistake, in His saying: “There is no blame upon you for what you have done by mistake, but [only] for what your hearts intended.” [al-Aḥzāb:5].
On that basis, his ʿUmrah is valid and nothing is required of him. The scholars of the madhhab have said: “The madhhab of the layperson is the madhhab of the one he follows.” And al-Shāfiʿī said: it is not disliked at any time.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.1