The winning percentage days indicates, the number of profitable days (where a positive percentage daily return was observed) within an investment period, as a percentage of the total number of trading days in the investment period. For example, a year-to-date analysis includes the total number of trading days from the beginning of the year until end-of-day yesterday. The winning percentage days would assess the number of profitable days within that year to date period, relative to the total number of trading days.
