Using Func instead of helper methods in a class
This is a perfect use case for an extension method. It gives maximum readability to your business code, and makes the method available on any DateTime object. 

//Test
class Program
{
    static void Main(string[] args)
    {
        Console.Write(DateTime.Today.MostRecent(DayOfWeek.Saturday));
        Console.Read();
    }
}
//Extensions
public static class DateTimeExtensions
{
    public static DateTime MostRecent(this DateTime date, DayOfWeek dayOfWeek)
    {
        var day = date.Date;
        while (day.DayOfWeek != dayOfWeek)
            day = day.AddDays(-1);
        return day;

    }
}