What's the earliest you can enter and purchase a day pass??

I'd reach to Mohonk Mountain House directly either with a phone call or via email.

The Preserve opens at 7am for members. Non members at 9 am

I don't know the exact time of earliest entrance..we entered at approx 10 am and we had plenty of time to do the lake rowing, the climb to the top and descend to the hotel for late afternoon tea and walk back to car park at approx 6/7 pm