I don't think traffic will be very different weekend/weekday - I suspect most tourist are like me and don't pay attention to the date/day, and there isn't a lot of local traffic on that road.
If you're just arriving, doing it the first day might work well. You'll be up very early Hawaii time, so any businesses/activities/etc won't be open. And if you're in Wailea/Lahaina/Ka'anapali, it takes an hour to get to the beginning of the good part. So leave at 5am and be past Paia in search of waterfalls not long after sun-up, beating almost all of the crowds.
It also gives you plenty of time. We left Ka'anapali just before 7a last week and still had to keep an eye on the clock to leave Kapahulu in time to beat sundown. I could easily have used more time on the Pipiwai Trail.
(Haleakala for sunrise is another 1st morning choice - 2nd morning you'll still be up early for Hana)