Anas (Peace be upon him) reported that upon arriving in al-Madinah, the Prophet saws found its people celebrating two days whose significance was held over from the Jaahiliyyah. The Prophet (Peace be upon him) said:

When I came upon you, you had two days that you continued to celebrate from the Jaahiliyyah; indeed Allah has substituted them for you with what is better: the day of Sacrifice and the day of Fitr (breaking the fast).

Shaykh Ahmad `Abdur-Rahman al-Banna said:

"(They are better because,) the day of Sacrifice and that of Fitr are legislated by Allah, and are His choice for His creatures. They follow the completion of two of the greatest pillars of Islam, Hajj (pilgrimage) and fasting. On these days, Allah forgives those who performed Hajj and who fasted, and He sheds mercy on all of His obedient creatures.

On the other hand, the days of Nayruz and Mihrajan were devised by the people of those times, because of good weather or other passing qualities.

The difference between the two cases is apparent for whoever ponders upon this."