There is a day in a year when descends the pestilence; at the end of the hadith Laith said that the non Arabs save themselves from it in Kanun Awwal (this is the month of December).
أَنَّهُ قَالَ فَإِنَّ فِي السَّنَةِ يَوْمًا يَنْزِلُ فِيهِ وَبَاءٌ وَزَادَ فِي آخِرِ الْحَدِيثِ قَالَ اللَّيْثُ فَالأَعَاجِمُ عِنْدَنَا يَتَّقُونَ ذَلِكَ فِي كَانُونَ الأَوَّلِ