Date_Of_Event.Date <= DateTime.Today.Date.AddDays(-1))
You have the comparison backwards
// This should work in LINQ-to-SQL / EntityFramework Date_Of_Event >= DateTime.Today.AddDays(-1) && Date_Of_Event < DateTime.Today
// This will work in LINQ-to-Objects or anywhere else, really. Date_Of_Event.Date == DateTime.Today.AddDays(-1)
As a side note, it may be a good idea to store the value of
DateTime.Today in a variable, so that weird bugs don't show up around midnight (or 11:00pm or 1:00am depending on daylight saving time).
Date_Of_Event.Date == DateTime.Today.AddDays(-1)
This will remove the time part of the date time, and keep only the date so you can check for equality instead of the time being inside the correct interval.