It depends on the college you are trying to apply to. You'll want to apply early if you are getting into a rolling admissions colleges because the earlier they can get you in, the more you'll be considered. It also depends on if you want to do some type of extracurricular over the summer after junior year/before senior year, such as a part-time job or a summer camp. If you are then you'll want to apply after that extracurricular is completed so it can enhance your application. Also, it depends on if you are ready to apply, if you have all your material and information together and all the additional documents. I would say to first look at what the colleges you want to apply for require in the application.
Hope this helps!
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