Hajj takes place both in multiple months and on specific days. The months are a period for entering the state of Ihram which is a constituent of Hajj. It can be entered in any of the three months.
الحج أشهر معلومات فمن فرض فيهن الحج فلا رفث ولا فسوق ولا جدال في الحج
Hajj is [during] well-known months, so whoever has made Hajj obligatory upon himself therein [by entering the state of ihram], there is [to be for him] no sexual relations and no disobedience and no disputing during Hajj.
— Quran 2:197
However days are fixed for specific rituals of Hajj such as the Day of Arafah, the Day of Nahr and the days of Tashriq. The prescribed rituals on these days can only be performed at their fixed days and there is no difference of opinion on that between the Malikis and others.
وأذان من الله ورسوله إلى الناس يوم الحج الأكبر
And [it is] an announcement from Allah and His Messenger to the people on the day of the greater pilgrimage
— Quran 9:3
ليشهدوا منافع لهم ويذكروا اسم الله في أيام معلومات على ما رزقهم من بهيمة الأنعام فكلوا منها وأطعموا البائس الفقير
That they may witness benefits for themselves and mention the name of Allah on known days over what He has provided for them of [sacrificial] animals.
— Quran 22:28
Also see Why Quran use 'Months' for period of Hajj?