+1 to what Dryer said.

December - Feb it can down into the low 20's at night, although usually it is in the mid 30's - 40's.

March - May things start warming up and night time temps will be 40 - 60.

June - Aug/Sep it is not unusual for temps to stay up into the 80's until 3-4am. Sometimes it will drop into the 60's if a front moves through.

So just as with clothing, you often have to layer your sleep system. I carry a fleece blanket sometimes during summer and I also have a Sportsman's Guide 50F bag that I use as a quilt.

As a general, overall recommendation I would go with a 20F bag knowing that most of the less expensive brands are generous with that rating.