Best way (for me) to explain the new Photos App is that it's essentially the iOS app, for Yosemite.
Looking at my application now, I do NOT see a "By Month" option. You are given the following breakdown options:
Photos
Shared
Albums
Projects
Albums gives you the typical breakdown of "All Photos", "Videos", etc... the first option (Photos) just breaks down photos by day. If you took 4 photos on March 4th, 2015 you'll see those under that date, then the 1 photo you took on March 6th, etc, etc.
My album is pretty disorganized though - I cannot say for certain that it wouldn't offer you these options if your photos were already in groups for each Month / Year.