The United States presidential inauguration day used to be on 4th March, but in 1937 the Twentieth Amendment changed the date of inauguration day to 20 January. If that day is a Sunday, inauguration day is moved to 21 January.
Has this day had any significant effect on the stock market?
The following chart plots the daily returns for the S&P 500 Index for inauguration day (ID) in the years from 1953 to 2009. Note: the chart only includes inauguration days for first terms (on the grounds that the market most likely knows what to expect with second-term presidents).
As can be seen shares have been weak on inauguration days. Since the 1963 inauguration of Lyndon B. Johnson the S&P 500 has been down on every inauguration day.
The following chart plots the average daily returns for the S&P 500 Index for the trading day before inauguration day, the day itself and the day after.
Since 1953 the average daily return for the S&P 500 on inauguration day has been -1.1%. For the day after ID the average daily return is 0.7%, so there does seem to be a partial relief rally afterwards.