When Allah—Glorious and Exalted—made Hajj specific known months, it is not permissible to enter into Hajj before its time, just as it is not permissible to enter into the prayers before their times.
Sayyiduna Ibn Abbas (may Allah be pleased with both of them) narrates that the Ihram for Hajj will only be worn during the months of Hajj. Because the Sunnah method of Hajj is that the Ihram for Hajj should only be worn during the months of Hajj.
Hadith Referenceصحيح ابن خزيمه / كتاب: المناسك / 2596