According to @StockMKTNewz, the S&P 500's single best session for each year from 2005 to 2025 ranges from +1.4% (2017-03-01) to +11.6% (2008-10-13), with notable spikes including 2025-04-09 at +9.5% and 2020-03-24 at +9.4% (source: X post, Jan 8, 2026: https://twitter.com/StockMKTNewz/status/2009305865266844040). Based on the posted list, the average best-day gain across the 21-year sample is about 4.29% and the median is 2.9% (source: X post by @StockMKTNewz, Jan 8, 2026: https://twitter.com/StockMKTNewz/status/2009305865266844040). The top days cluster in January and March with 4 occurrences each, followed by June, August, November, and December with 2 each, highlighting early-year seasonality in big upside moves (source: X post by @StockMKTNewz, Jan 8, 2026: https://twitter.com/StockMKTNewz/status/2009305865266844040). Six years saw best-day gains of at least 5% (2008, 2009, 2018, 2020, 2022, 2025), underscoring the tail-risk contribution of a few outsized sessions to annual returns (source: X post by @StockMKTNewz, Jan 8, 2026: https://twitter.com/StockMKTNewz/status/2009305865266844040). For crypto market participants, this calendar can serve as an actionable watchlist to monitor cross-asset risk sentiment around historically active windows when U.S. equities printed their largest daily gains, particularly in January and March (source: X post by @StockMKTNewz, Jan 8, 2026: https://twitter.com/StockMKTNewz/status/2009305865266844040).