The given code calculates the last Friday before (or on)...

The given code calculates the last Friday before (or on) a date represented by lastDay. Here's what each part does:

Code:

var lastFriday = lastDay.AddDays(-(int)lastDay.DayOfWeek - 2);

Explanation:

1. lastDay:
   This is a DateTime object, representing a specific date.

2. lastDay.DayOfWeek:
   DayOfWeek is an enumerated property of the DateTime class in C#. It tells you the day of the week for the lastDay date (e.g., Sunday = 0, Monday = 1, ..., Saturday = 6).

3. (int)lastDay.DayOfWeek:
   Casting the DayOfWeek enum to an integer converts it into its numeric representation — Sunday = 0, Monday = 1, ..., Saturday = 6.

4. -(int)lastDay.DayOfWeek - 2:
   This calculates the number of days to subtract from lastDay to get to the most recent Friday:

   • First, negate the integer value of the current day of the week (-(int)lastDay.DayOfWeek).
   • Then subtract 2 to ensure it lands on the nearest prior Friday.

   For example:

   • If lastDay is a Sunday (DayOfWeek = 0), the subtraction becomes -0 - 2 = -2 (2 days back to Friday).
   • If lastDay is a Wednesday (DayOfWeek = 3), the subtraction becomes -3 - 2 = -5 (5 days back to Friday).
   • If lastDay is already a Friday (DayOfWeek = 5), the subtraction becomes -5 - 2 = -7, which lands on the previous Friday (the formula assumes a strict "last Friday").

5. lastDay.AddDays():
   The AddDays() method adjusts the lastDay date by the number of days provided (negative values subtract days). This moves lastDay backward to the nearest Friday.

Result:

• The variable lastFriday now holds the DateTime value of the most recent Friday (before or on the date stored in lastDay).

Example:

If lastDay is 2023-10-11 (Wednesday):

• lastDay.DayOfWeek = DayOfWeek.Wednesday, which is 3.
• (int)lastDay.DayOfWeek = 3.
• -(int)lastDay.DayOfWeek - 2 = -3 - 2 = -5.
• lastDay.AddDays(-5) subtracts 5 days from 2023-10-11, resulting in 2023-10-06 (Friday).
  Thus, lastFriday will be 2023-10-06.

Summary:

This code calculates the last Friday prior to (or including) the given lastDay.