Saturdays and Sundays. There is also a great Brooklyn Bridge Tour (Free Tours by Foot) that ends in this area.
Every Saturday and Sunday.
These are great tours, I have been on a few and have had great experiences! You just tip the guide what you feel the tour was worth. I usually do about 20/ per person. Still much cheaper than a traditional walking tour.
If you have never heard of the High Line, it is a park that was made on an old elevated train track running through Manhattan.
The Met is actually free for residents of NYS with a donation.