Sundays and Thursdays are heavy travel days (arriving and leaving the island) so the road to Hana may be less crowded. Start early to get the most out of your day. This is one of the best things to do in Maui on a budget.
Is Road to Hana busier on weekends?
I find that Saturday and Sunday are better days in general, since many of the weekly rentals and vacation packages begin on Sat or Sun. Traffic to and from the airport is heavier on those days, but we notice a definite slowdown in foot traffic and beach goers, road to Hana travellers, etc., especially on Sundays.