CalendarHelper.cs

/// <summary>
/// Helper for working with calendar concepts.
/// </summary>
public static class CalendarHelper
{
	/// <summary>
	/// Returns all of the weeks for specified calendar month.
	/// </summary>
	public static IEnumerable<Week> GetWeeksForCalendarMonth(DateOnly month)
	{
		var firstDayOfMonth = new DateOnly(month.Year, month.Month, 1);
		var lastDayOfMonth = firstDayOfMonth.AddMonths(1).AddDays(-1);
		var weeks = new List<Week>();
		for (int dayOfMonthIndex = 1; dayOfMonthIndex <= lastDayOfMonth.Day; dayOfMonthIndex++)
		{
			var currentDate = new DateOnly(month.Year, month.Month, dayOfMonthIndex);
			var days = new List<Day>();
			for (int dayOfWeek = 0; dayOfWeek <= 6; dayOfWeek++)
			{
				var dayOfWeekOffset = dayOfWeek - (int)currentDate.DayOfWeek;
				var dayDate = currentDate.AddDays(dayOfWeekOffset);
				days.Add(new Day(dayDate));
				if (dayOfWeekOffset >= 0)
				{
					dayOfMonthIndex++;
				}
			}
			weeks.Add(new Week(days));
		}
		return weeks;
	}
}

public readonly record struct Week(List<Day> Days);

public readonly record struct Day(DateOnly Date);