Though trekking to Mardi Himal is double throughout the year, some seasons are the best time to visit there. Autumn and Spring are the Best Time for Mardi Himal Trekking due to stable and clear weather. But this season might not best for those who are looking for less busy(less crowded) crowded seasons. As Autumn and Spring are the best seasons, there will be huge traffic in Mardi Trekking. If you want to avoid this somehow, you can make the trip some weeks ago before this season starts or some weeks later after the best seasons.
In September, Early September might not perfect(depends on weather) to visit this trail due to post monsoon. However, generally, we can get good weather from mid of September. You can see the best view of Annapurna and the fishtail. Moreover, it is not a busy season. Hence you will get more hospitality from the teahouses.
December is also considered the best time to trek due to favorable weather for viewing the sights. It will be cold in the morning and evening outside of teahouses but during the day, the temperature might be suitable to hike the base camp of Mardi. Traffic will get less than the peak(October to November). Hence December is also the best time for Mardi Himal Base Camp Trekking.
So in conclusion, if you are looking for the best time to visit Mardi Himal in terms of weather as well as less traffic, Mid of February to Early March and Mid of September to Early October is the best month to visit Mardi Himal Base Camp Trekking.