Amir al-Muminin, Imam Ali (pbuh)
إنَّمَا هُوَ عِيدٌ لِمَنْ قَبِلَ اَللَّهُ صِيَامَهُ وَ شَكَرَ قِيَامَهُ وَ كُلُّ يَوْمٍ لاَ يُعْصَى اَللَّهُ فِيهِ فَهُوَ عِيدٌ
It is an Eid for him whose fasting Allah accepts and of whose prayers He is appreciative; and (in fact) every day wherein no sin of Allah is committed is an Eid
Nahjul Balagha, saying no. 428
