This code calculates the date of the most recent Friday...

This code calculates the date of the most recent Friday relative to a given date stored in the variable lastDay. Here's a breakdown of what the code does:

1. lastDay.DayOfWeek:
   This retrieves the day of the week (as an enumerated DayOfWeek value) for lastDay. The DayOfWeek enumeration maps days to integer values (Sunday = 0, Monday = 1, ..., Saturday = 6).

2. -(int)lastDay.DayOfWeek:
   Converts the DayOfWeek value of lastDay into its integer value, then negates it.
   For example:

   • If lastDay is a Monday (value of 1), -(int)lastDay.DayOfWeek becomes -1.
   • If it's a Sunday (0), this value is 0.

3. -(int)lastDay.DayOfWeek - 1:
   The code subtracts 1 from the negated value of DayOfWeek, effectively going one day earlier than that day. This ensures the calculation finds the Friday of the previous week if lastDay falls after Friday.

4. lastDay.AddDays(...):
   Using the result of the calculation -(int)lastDay.DayOfWeek - 1, the code adjusts lastDay backward by that number of days to arrive at the most recent Friday.

Summary

The code calculates the most recent Friday before (or on) the date stored in lastDay.

For example:

• If lastDay is Saturday, it moves back 1 day to Friday.
• If lastDay is Tuesday, it moves back 4 days to the previous Friday.
• If lastDay is Friday, the result is the same day since it’s already Friday.