I have gone to Thermas many times over the years at different times of the year.
It is difficult to predict a "best" day at Thermas.
The only times I experienced a night at that sauna with fewer available guys were nights when it rained or when the weather was especially cold outside.
Despite the weather not being the best, the sauna still draws a fair selection of available guys.
The key being, rain and cold are the nights you can expect to see fewer guys than nights when the weather is nice.