Broccoli is a cold weather plant. When broccoli plants are added to the garden, it takes about 60 or 70 days from the time a plant is put into the ground until the time broccoli is ready to be harvested (add about 25 days if started from seeds).
Broccoli produces well in cool temperatures (when temperatures do not exceed 70 degrees Farenheit) but can be damaged when temperatures reach the mid 20’s.
Therefore, in our area (USDA zone 8b), planting in late August to Early September ensures a harvest before the truly cold temperatures arrive (usually around late December or mid-January). We planted late in September due to the heat and dry conditions (a mini-drought) experienced from then end of May until early October (in 2015, we experienced the wettest May and October on record, with no rain and much heat from June through September).
Now it is time to enjoy the limited harvest we have (many of the plants died from the unusually high temperatures in September/October).