I don't think VS can take any credit for his commments about the first few trading days of the month. The original research was done by Norman Fosback and Arthur Merrill, and is described in Gary Smith's excellent book, "How I Trade For a Living."
This explains that the last trading day of the month and the first 4 trading days, plus the 2 trading days before a market holiday are usually +ive. Between 1928 and 1994, if you had only held the S&P500 for those days, representing 28% of all trading days, you would have turned $10k into $4.6million. If instead you had invested for the remaining 72% of days only, you would have turned $10k into $569 by the end of 1994. That's not a typo!
Recent research has updated this to include the 2nd to last day of each month, and the 5th trading day of each month, but excluded if these are the first trading day of the week.
Additionally, the period between 2 days before thanksgiving and ending on the 5th trading day of Jan accounts for 40% of the growth in the S&P500 in the last 70 years. The month end and pre-holiday trading days lieing within this 7 week period account for 45% of market entire price return over that 70 year period. Mind boggling stuff!