Suppose an economics forecaster discovers that on days when the sunspot count is high stock market on the following day is bullish, that is stock market prices edge upwards

In addition, he also observes that on days with a low sunspot count the following day the stock market tends to be bearish, that is stock market prices tend to fall. The forecaster then concludes that there is a positive relationship between the sunspot count and stock market prices and proceeds to base his investment decisions on this premise. What kind of an error has this forecaster made?

The forecaster has fallen into the post hoc fallacy. Simply because one set of events precedes another is not enough to say that one is the cause of the other.