You did a pretty good job !!!!
I would shift the graph to the right by pi/6 so we get
y = -20cos (pi*x / 6 - pi/6) + 51
It lines up a little better this way.......the temps will not be exact for all months because we don't have constant changes between them, but this is about the best you can do with a periodic trig function
See the graph here with a few selected points : https://www.desmos.com/calculator/zxwgqqtjzl
