Generally once in the late spring (which is usually Memorial Day for most of the US, but it might be late April for you), and twice in the fall. First fall app would be around Labor Day and the second one would be around Thanksgiving.

Why are you using weed n feed? Do you have weeds? Because you shouldn't. St Aug usually grows dense enough to keep weeds out...unless you are not fertilizing. Or if you are watering more than once per week. By and large it is much better to fertilize with a good fertilizer and spot spray weeds with a liquid. St Aug requires a liquid with Atrazine as the normal stuff (Weed-B-Gone) is not good for St Aug.

If you want to use organic fertilizer, you can do that any day of the year, but those three days I mentioned are still good for a rule of thumb. I've been organic since 2002 and don't keep up with chemical fertilizers. As for organic fertilizers, plain alfalfa pellets work excellent.

Milorganite.

Thats all you need to know. Whenever and how much you like. Thats it.