Number of Weeks in a Month Vb.Net - vb.net

Question

Is there any DateTimePicker function to get the number of full weeks in a month?
Im looking for something like:
WeeksInMonth("01/01/2020") = 5
This will be the number of weeks per month manually calculated of this year:
Corrigendum: September has 4 weeks.
EDIT:
If the month starts in the mid of a week it should count as full week. It should also counts as full week if the month ends in the mid of a week.
From Monday to Sunday.
EDIT 2:
By mid week i mean if the week starts in Thursday it considers it as a complete week, because it was 4 days (thursday,friday,saturday and sunday) out of 7.
Same for the end, if the week end in thursday it will count as a complete Week, if the week finish in Monday, Tuesday or Wednesday it wont count because it only has 3 days out of 7.
Im taking in to consideration that my complete week starts in Monday and ends in Sunday.
Ex. January 2020 First complete week is: 30-dec-2019 to 05-jan-2020.
Another Example will be March 2020, Last complete week for me is 23-Mar-2020 to 29-Mar-2020, the 2 remaining days (30,31) would be part of April 1st complete week.

Answer 1

According to your logic, a week is in a month as long as its Thursday is in that month. If Thursday is in a month, then so are at least Monday, Tuesday, and Wednesday (if at the end) or Friday, Saturday, and Sunday (if at the beginning). So if you count the number of Thursdays in a month, you should get the number of weeks in that month.
Simply use LINQ to get all the days in a month, then take only days which are Thursday, then count those days
Public Function GetNumberOfWeeksInMonth(year As Integer, month As Integer) As Integer
Return Enumerable.Range(1, DateTime.DaysInMonth(year, month)).
Select(Function(d) New DateTime(year, month, d)).
Where(Function(d) d.DayOfWeek = DayOfWeek.Thursday).
Count()
End Function
' Concise version with Count with predicate
Public Function GetNumberOfWeeksInMonth(year As Integer, month As Integer) As Integer
Return Enumerable.Range(1, DateTime.DaysInMonth(year, month)).
Count(Function(d) (New DateTime(year, month, d)).DayOfWeek = DayOfWeek.Thursday)
End Function
Call it
For i = 1 To 12
Console.WriteLine($"{New DateTime(2020, i, 1):MMM}{vbTab}{GetNumberOfWeeksInMonth(2020, i)}")
Next
Jan 5
Feb 4
Mar 4
Apr 5
May 4
Jun 4
Jul 5
Aug 4
Sep 4
Oct 5
Nov 4
Dec 5

Answer 2

I assume this is something similar to the ISO 8601 scheme for year-weeks, where a week (a Mon-Sun period) falls into the year which contains it's Thursday.
Here a month contains a week if the Thursday of that week falls in the month.
Logically the number of such weeks in a month is the same as the number of Thursdays in that month.
You can derive this figure from the number of month-days in the month, and the week-day of the first month-day of the month.
A 28-day month always contains exactly 4 Thursdays. Months with more than 28 month-days may contain 5 Thursdays.
You can convert an ISO week-day number (Mon = 1, Sun = 7) to the following format like so: ((weekday + 2) Mod 7) - 2
Mon = 1, Tue = 2, Wed = 3, Thu = 4, Fri = -2, Sat = -1, Sun = 0
Then add this adjusted week-day figure to the number of month-days. If the result exceeds 32, then it is a month containing 5 weeks.
So a 31-day month starting on a Monday, produces a figure of 32 (31 + 1). It contains 4 weeks.
A 31-day month starting on a Tuesday, produces a figure of 33 (31 + 2). It contains 5 weeks.
A 31-day month starting on a Friday, produces a figure of 29 (31 + (-2)). It contains 4 weeks.
Hope this helps.