The time of year is useful, but more useful is the nighttime low temperatures you expect to encounter. When calculating that temperature, be sure you figure out what the low will be where you are camping, not what it will be in the nearest town. Sometimes, you can extrapolate: for example, if you're going to be camped on top of a mountain, 2500 feet higher than the town, subtract 5 or 10 degrees. Being out in the country is also good for a little less warmth.

For example, when I'm hiking in August, let's say I know it's going to probably be a nighttime low in the 60's in town (per the weather channel.) However, I'm going to be camped a hundred feet lower, in a narrow valley alongside a decent-sized creek. Those conditions are likely to yield a microclimate that creates lows in the 50's where I'll be. In that case, a blanket may not be enough if I'm sleeping under the stars - but it may be if I'm in a tent. And two blankets might also be enough.

A sleeping bag isn't always necessary - when I've known the lows were going to be in the mid-70s, I've successfully used a fleece or wool blanket. Usually, however, except in July and August in the Ohio Valley, I always carry a 40 degree bag. I may not start out under it, but along about 3:30 in the morning, it usually feels nice to pull it over me like a quilt. (It's a Western Mountaineering MityLite: semi-rectangular, hoodless down bag that zips fully open - a very versatile piece of gear since I can use it as a bag or a quilt, depending on temperatures encountered.

REI sells a Travel Sack that's a 55-degree bag, 27 ounce total weight, with a "flat" hood (as opposed to a shaped hood) that only costs $60. I got one for a niece who was only backpacking with me in the summer, and it was perfect. It's got just a little synthetic insulation in it, and a nylon shell (keeps leaves and twigs from sticking to it.) She used it the same way I use mine: mostly as a quilt, but occasionally (when that unanticipated dip in temps hit) as a full-out sleeping bag. One time, when it got cooler than we expected, she slept in her pile jacket inside that bag and stayed plenty warm. The Travel Sack may be just what you're looking for. (Use the link at the left to go to REI.)