April is the peak month for the Peony Festival, which draws enormous crowds and significantly raises hotel prices. If you want to see the peonies at their best, mid-April is the time — but book accommodation weeks in advance. Outside festival season, Luoyang is far less crowded and considerably cheaper.
Spring (March–May) and autumn (September–November) are the most comfortable seasons. Summer is hot and humid, with July and August temperatures regularly exceeding 35°C. Winter (December–February) is cold and grey, though the Longmen Grottoes and White Horse Temple are almost empty and very atmospheric in frost or light snow.
The Chinese national holidays — Golden Week in early October and Spring Festival (Chinese New Year, January or February) — bring heavy domestic tourism. Avoid these dates unless you enjoy crowds or have no choice.