根据日期计算出周一和周日

DateTime baseDate = DateTime.Today;

var today = baseDate;
var yesterday = baseDate.AddDays(-1);
var thisWeekStart = baseDate.AddDays(-(int)baseDate.DayOfWeek);
var thisWeekEnd = thisWeekStart.AddDays(7).AddSeconds(-1);
var lastWeekStart = thisWeekStart.AddDays(-7);
var lastWeekEnd = thisWeekStart.AddSeconds(-1);
var thisMonthStart = baseDate.AddDays(1 - baseDate.Day);
var thisMonthEnd = thisMonthStart.AddMonths(1).AddSeconds(-1);
var lastMonthStart = thisMonthStart.AddMonths(-1);
var lastMonthEnd = thisMonthStart.AddSeconds(-1);