Bilma is in the deep Sahara, where summer heat regularly tops 45°C (113°F) and the nights stay warm. Travel between June and September is genuinely dangerous and most tour operators do not run trips at all in this window.
The window is November to February: daytime highs around 25–30°C, cold clear nights that can drop near freezing, and the harmattan wind blowing thin desert dust across the south. The salt caravans are most likely between October and January. Avoid the shoulder months either side of summer — March and April get too hot for comfortable trekking, and dust storms become more frequent.
Before going anywhere in northern Niger, check your government's travel advisory. The region is regularly classified as do-not-travel for security reasons; current conditions matter more than any seasonal weather window.