"Nay, [it was] the plot of the night and the day," that is: your speech to us by night and by day, "when you enjoined upon us to disbelieve in Allah and to attribute to Him equals," meaning: their idols which they set as equals to Allah, worshiping them instead of Him.
﴿بل مكر اللَّيْل وَالنَّهَار﴾ أَي: بل قَوْلكُم لنا بِاللَّيْلِ وَالنَّهَارِ ﴿إِذْ تَأْمُرُونَنَا أَنْ نَكْفُرَ بِاللَّه ونجعل لَهُ أندادا﴾ يَعْنِي: أوثانهم عدلوها بِاللَّه فعبدوها دونه