Asyut has a hot desert climate. Winter (November to February) is the only really comfortable season — clear skies, daytime highs in the low 20s Celsius, cold nights. Summer is brutally hot, with afternoons regularly above 40°C and dust storms in spring.
If you can, time a visit to the Holy Family moulid at Durunka in August (huge crowds, oppressive heat but extraordinary atmosphere) or the gathering at Deir el-Muharraq in late June. Otherwise, December and January are the best months.