For cool season grasses, most of your fertilizing should be done in late summer and fall. A good way to remember dates is Labor Day, Halloween, and Thanksgiving. Now these dates are location specific, if you are in Maine they will be earlier, later for a location like Virginia. If you are using synthetic fertilizers the first two dates shouldn't be all quick release nitrogen, 50:50 is a good ratio. Lesco carries 25-0-6 which is 50:50. Apply one pound of nitrogen per 1000 square feet. The first number is the percentage of nitrogen per pound. So if you have 1000 square feet, you will need to apply 4 lbs of 25-0-6 since one pound of this fertilizer is 25% N. The second number is for phosphorus (P) and the third is for potassium (K). P is a relatively stable nutrient and doesn't leach of of soils readily, so if your P levels are good, you don't need to add any unless you are seeding. K levels are more stable than N, but they do leach, and most fertilizers have some K to replenish what is used and lost. A good soil test is a good idea to see where your nutrient levels are (www.loganlabs.com $20 basic soil test). The last fertilizer drop of the season should be when top growth has stopped, but the ground isn't frozen. For this application you should use all quick release fertilizer. Quick release nitrogen comes in the form of Urea, Ammonium nitrate, ammonium sulfate, and ammonium chloride. It is usually listed as quick release or water soluble. Water insoluble is slow release, and is usually found as sulfur coated urea. Whenever you apply fertilizer is always a good idea to water it in to prevent fertilizer burn.

What regiment to follow depends on the type of grass you have. Since you're in Canada, I assume you're growing a cool season grass like KGB or fescue.

tiemco's advice is good. The only thing I would add is you don't want to fertilize if it's hot and dry. Most fertilizers are salts and if it's too hot and/or too dry, you can do more damage than good. I would wait until the summer temps are done with and make sure there's water either through rain or irrigation. I would never fertilize where I live in Maryland on labor day. It's still summer time here. Where you live, it may be OK.