The Altcoin Season Index has decreased to 17 as of December 26, indicating a continued period of Bitcoin dominance over altcoins. This marks a slight decline from the previous day's index of 18. The index, which measures the performance of the top 100 altcoins against Bitcoin over a 90-day period, had previously peaked at 78 on September 20. Currently, only 17 altcoin projects have outperformed Bitcoin within this timeframe, according to CoinMarketCap data.