The best time to visit Abeche is November through February, when temperatures are tolerable and the dust from the harmattan is manageable. Days are warm, nights can be genuinely cold, and the roads are at their most passable. This is also the window when overland expeditions to the Ennedi Plateau depart from Abeche, so the town has more travelers and a slightly livelier atmosphere.
March to May brings brutal heat. Temperatures regularly exceed 40°C and the landscape becomes punishing. Unless your plans require it, this is a poor time to be in eastern Chad.
The rainy season (June–September) brings some relief from the heat but can cut Abeche off from the rest of the country. Roads become unreliable and movement through the eastern desert is difficult.
Ramadan significantly changes the rhythm of the city. Markets open and close at unusual hours, restaurants may not serve during daylight, and the evening breaking of the fast turns the streets festive. It is not a bad time to visit if you come prepared, but logistics require more flexibility than usual.