I mean Monday,Tuesday,Wednesday...etc.Volume tends to lower on Monday,Friday usually closes higher,Monday and Tuesday seem more active,etc..
I do DoW / ToD analysis (DoW : Day-of-Week, ToD : Time-of-Day) on my strategies, and use it to filter out unreliable time windows. Be a little cautious, because results might not be statistically significant, if using too short period of time for analysis.
Recurring periods of time during which a strategy produces results "far away" from its nominal results.