The day of Bu'ath was a day (i.e. battle) which Allah caused to take place just before the mission of His Apostle so that when Messenger of Allah ﷺ came to Medina, they (the tribes) had divided (into hostile groups) and their nobles had been killed; and all that facilitated their conversion to Islam.  

كَانَ يَوْمُ بُعَاثٍ يَوْمًا قَدَّمَهُ اللَّهُ ﷻ لِرَسُولِهِ ﷺ فَقَدِمَ رَسُولُ اللَّهِ ﷺ الْمَدِينَةَ وَقَدِ افْتَرَقَ مَلَؤُهُمْ وَقُتِلَتْ سَرَاتُهُمْ فِي دُخُولِهِمْ فِي الإِسْلاَمِ  

The day of Bu'ath (i.e. Day of fighting between the two tribes of the Ansar, the Aus and Khazraj) was brought about by Allah for the good of His Apostle so that when Messenger of Allah ﷺ reached (Medina), the tribes of Medina had already divided and their chiefs had been killed and wounded. So Allah had brought about the battle for the good of H is Apostle in order that they (i.e. the Ansar) might embrace Islam.  

كَانَ يَوْمُ بُعَاثَ يَوْمًا قَدَّمَهُ اللَّهُ لِرَسُولِهِ ﷺ فَقَدِمَ رَسُولُ اللَّهِ ﷺ وَقَدِ افْتَرَقَ مَلَؤُهُمْ وَقُتِلَتْ سَرَوَاتُهُمْ وَجُرِّحُوا فَقَدَّمَهُ اللَّهُ لِرَسُولِهِ ﷺ فِي دُخُولِهِمْ فِي الإِسْلاَمِ  

Allah caused the day of Buath to take place before Messenger of Allah ﷺ was sent (as an Apostle) so that when Messenger of Allah ﷺ reached Medina, those people had already divided (in different groups) and their chiefs had been killed or wounded. So Allah made that day precede Messenger of Allah ﷺ so that they (i.e. the Ansar) might embrace Islam.  

كَانَ يَوْمُ بُعَاثٍ يَوْمًا قَدَّمَهُ اللَّهُ لِرَسُولِهِ ﷺ فَقَدِمَ رَسُولُ اللَّهِ ﷺ وَقَدِ افْتَرَقَ مَلَؤُهُمْ وَقُتِّلَتْ سَرَوَاتُهُمْ وَجُرِّحُوا قَدَّمَهُ اللَّهُ لِرَسُولِهِ ﷺ فِي دُخُولِهِمْ فِي الإِسْلاَمِ