Due to the time-sensitive and extremely fickle nature of the stock market, it is impossible for even expert investors to determine at what point it is best to buy or sell, except in hindsight. The best time also depends upon the stock and the needs of the investor.
The behavior of individual stocks is notoriously difficult to forecast, and the stock market changes from day to day, so it is impossible to give unilateral advice on buying at any given point in time. The general advice is to buy low and sell high.
Some investors, such as journalist Jim Kramer of CNBC, have more or less complex strategies for determining when it is best to buy, though even they admit that their strategies are not appropriate for all investors under all circumstances. Depending upon one's investment goals and financial situation, the best time to buy may be very different from the best time proclaimed by someone with completely different goals and finances.