It really depends on your region and which brands you're looking for. Best Buy and Costco are two totally different worlds - with Costco you have to be a member and their stock changes constantly, so if you find what you want at the right time that's great, but it's not guaranteed. Best Buy generally has more selection and regular sales. Then there are also appliance specialty stores that have some interesting deals, or even sites like Amazon Canada that are worth checking out.
For timing, Black Friday and Boxing Day are obviously the big moments for appliance discounts - you can save quite a bit then. But honestly, I've noticed that good deals come around pretty regularly throughout the year, like at the end of the month when stores want to clear out stock. My advice is to compare prices online before going to a store, and don't rush - once you know exactly which fridge and stove model you want, you can track the prices for a few weeks. Often a sale pops up when you least expect it.