I have setup automatic withdrawal with my brokerage to purchase shares of an S&P500 Index fund the 8th of every month. When I first caught wind of the USA credit downgrade Saturday morning, I started thinking "... maybe I should change my automatic purchase date of the S&P500 Index fund to later in the week". Sure enough, my purchase at opening today lost about 3.5% of its value within a few hours.
Given that you will certainly purchase $X of an index fund every month, do you micro-manage the exact day of the month to purchase that fund based on the news?