Ultimately, there is no "bad" time to upgrade your attic insulation, but there are several key reasons why the summer is a very good time to get the job done.
First, most people wait until the first big cold snap of winter, before they start thinking about upgrading their attics. It's a bit like snow tires, except, its probably a bad idea to put snow tires on in the summer, but you could probably do without the high prices, and long waits, to get them on when the snow does start to fly. The smart people, book their snow tires, and pay for them in the summer or fall. Unlike snow tires, attic insulation is easilly installed during the summer, and because it's not the rush season, the prices are more competitive, and the booking timelines are shorter.
A second reason to upgrade during the summer, is simply beating the heat. Insulation serves a dual function, in that it will keep your home warmer in the winter, and cooler in the summer. If you have air conditioning, it will keep the cool air in, and reduce how often your air conditioning has to work. With electricity being a growing commodity cost, this will definitely save you money.
Even if you don't have air conditioning, the added insulation will reduce how the heat in your attic, drops back down into your home, in the evenings when the hot summer sun finally recedes. If you don't have adequate insulation, that heat will drop down into your home more evidently, and can cause sweltering discomfort.
A third reason, associated with the heat, is the improved air flow you get with additional insulation, and an the evaluation of your attic ventillation that comes with it. Many of Calgary's homes lack adequate ventillation in their attics, and are at risk of shingle damage from overheated roof sheathing. This can significantly reduce the live of your roofing materials, and cause heat damage to the framing members below.
A fourth reason, is the added conveinence of having the work done during the summer months. You don't have your front door open when it's 30 degrees below zero, and you don't have the likelyhood of tracking dirty snow into your home. It's simply a more comfortable, and stress free environment for our workers, and the home owner.
If you are renovating your home, even in the interior, you may wish to have your attic done at the same time, while you are in renovation tolerance mode, that brief time period where you are willing to tolerate people coming into your home all day long, and making a bit of a mess, and making a little noise. You might as well get the attic done at the same time, never have to worry about it again.