I was there on a Saturday a couple of weeks ago, and they were available as soon as the gates were opened - about 9:30AM. In fact, we purchased the passes, then proceeded to Southwest Territory for walk-on rides on Raging Bull and Giant Drop - we had 5 rides by 10:30. When 11:00 rolled around, the park began to fill, then we made use of the FastLane passes.
If you stop to buy FastLanes, you will be deep in a line for V2, because everyone runs to V2 instantly. V2's queue will fill up while you are buying your passes. Not to worry - that is what FastLanes are for, to make that V2 wait shorter. Also, if FastLane gets cut off for a time at V2, or it is broke down, you can come back later and try it again with little time wasted, and there are other diversions in the V2 area to keep you busy (Batman, Iron Wolf, Logger's Run). Stick with my above listed strategy - very few run to SW Territory right away anyway because of V2. FastLane is highly recommended for V2 (even on weekdays), as breakdowns are somewhat common, so you don't waste your day waiting, then have it break down. Get them as soon as possible upon your entry to the park, they are limited, and are highly desirable to have on a busy day.
The FastLane booth is located in Hometown Square - when you enter the park, go to the right side of the carousel into Hometown Square and follow the signs to the FastLane booth. If you enter the park this way, you'll have no trouble finding it.
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