It depends on where you will be. In the Rockies or other high mountains it can freeze at night any month of the year. Even in the Appalachians it can get chilly at night even in July and August. In the desert it can get cold at night. So most of my tours I take the same bag that I use down to the mid teens F. Fortunately for me that is my Mountain Hardwear Phantom 45 that weighs just 17 ounces. I apparently put out heat like a furnace and I add a layer of clothes for colder weather.

I start without the bag when it is hot and pull it over me as it gets cooler leaving limbs out as required. As it gets cooler yet I climb in, then zip up, then pull the hood drawstring.

If you are camping somewhere there is no chance of it getting cold ,a silk liner bag or just a half of a bed sheet might work. In a pinch you can put on all of your clothes. Me, I am seldom on a trip where there is no chance of it getting fairly cold. We have had 110F and frost within a 48 hour period (and maybe even within a 24 hour period).

For a sleeping pad I always use my NeoAir regardless of temperature. It is light, comfortable, and packs small. For real winter I'd still use it, but add a foam pad.