The Santa Claus Rally was first published in the Traders Almanac in 1972. The definition is as follows: A stock market rise during the last 5 trading days in December and the first 2 trading days in the following January. For 2020 this means the Rally would start thursday December 24th, 2020; and end tuesday January 5th, 2021.
According to the 2019 Stock Trader's Almanac, stock prices have historically risen 76% of the time over the 7 trading days. So historically, there is bullish sentiment over these trading week.
There is a caveat, though; and it is well explained with the following quote from Yale Hirsch himself: "If Santa Claus should fail to call; Bears may come to Broad & Wall". Meaning, if the Santa Claus rally turn into a sell-off; January will be set for a continuation of selling.
December lows are currently the fundamental support level.