The best time to visit Sonauli is during the winter months, specifically from October to March, when the weather is most pleasant.
During this period, the temperatures are moderate, typically ranging from 10°C to 25°C, making it comfortable for exploring the local area and the nearby border crossing into Nepal. You'll find clear skies and less humidity compared to other times of the year.
This time of year is particularly good for travellers who wish to explore the religious and historical sites in the region. The cooler weather makes day trips to places like Lumbini, the birthplace of Buddha, much more enjoyable.