Bitcoin finished September 2026 at +6.5%. If you held only the best days, the return jumps to +24.8%, while missing them drops it to -14.7%. That tells you the month was driven by a handful of outsized sessions rather than smooth trend strength. The biggest upside day was 9/21/26 (+6.4%), and the roughest day was 9/15/26 (-3.7%) $BTC