Week days are best.
No swimming is allowed yet.
If you go to the park's website they'll have more up to date information about swimming/ boating. I believe after Memorial Day the area by the lake is open for recreational activities like swimming but I would definitely check their website to verify. There is also a fee to park at the main lake (I think it's $10). The park is pretty well maintained and the weekends the main area by lake can get crowded but the trails are often empty especially during the week.
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