CLL is right on. Applying fertilizer without a soil test is called "the shotgun" approach. It is like reaching in the medicine cabinet and grabbing any old medicine when you don't feel well.
And, since you stated you have weeds, I would strongly reccomend applying a herbicide with liquid fertilizer. Another benifit to adding herbicide to fertilizer is that less herbicide is needed. Fertilizer has a multiplying effect on herbicide.That way you pay for the application once and accomplish two things. This will save you money regardless of whether you do the application or hire it done.
Typically, herbicides (and fertilizer) are applied in the spring, when both weeds and grass are just starting to grow thus killing the weeds early before they rob valuable moisture and nutrients. This also will allow the desirable forage more moisture and nutrients for future growth.
Since you missed the spring application, I would consult you county agent or a commercial applicator for advice.