You can wing it...better that time than earlier in the summer, but you may run into troubles along the way and not get the nicest/best priced accomodation. When I was a kid we'd go on holiday with no reservations but ofetn ended up at cheap tacky motels.
Now I plan my trips in detail, as it's important to me to have a nice place to stay.
On one trip as a university student, at that time of year, we landed in Vancouver with no place to stay reserved and it took the better part of an afternoon/evening to find something in our price range. We ended up a ways out of the city.
You might want to reserve in Vancouver, expecially, and other tourist hot-spots, like Banff and the Okanagan, and wing it in between. This might still allow you some flexibility in your itinerary.

Thanks...Any suggestions as to what not miss and how to plan out the 12 plus days???

Where are you coming from and what do you like to do (city things, hiking, relaxing on a beach)? Are you landing in Calgary and departing from Vancouver, or vice versa, or do you need to land and depart in the same city (in other words do a circle route)? Do you like to stay put for a few days at a time or is moving each day OK?
If you can provide some more info you'll get more suggestions.

My not-to-miss things would be Banff, Lake Louise and Morraine Lake over at least a couple of days, the Icefields Parkway to Jasper, perhaps a day in Kelowna doing a wine tour, at least 3 days in Vancouver and a night or 2 in Victoria. Depending on your interests and departure city, I'd add in a Gulf Island visit or a couple of days in Tofino, or substitute these last 2 for some of the other stops.

Definately book in Vancouver. They hol a fair there in late August. i went impusively one year and ended up dragging between YWCA AND YMCA.

VANCOUVER NOT TO MISS: STAN;LEY PARK. rent bikes, sunbathe, swim. try to get to a smaller island, like Denman. I found info on walking tour guides. It was a very reasonable price and the guide, a university environmentalist student, very informative. There is a great market, arts and crafts exibition on ? Island in center of Vancouver. check out Chinatown. Victoria on Vancouver Island also excellent, museum, hike around Sooke. Butchart gardens. And That's only the west coast!