Quick way to determine days in date-range

Add Dateutils to your uses clause
and then use DaysBetween which requires two TDateTime values.
Here's what the helpfile says

[Delphi] function DaysBetween(const ANow: TDateTime; const AThen: TDateTime): Integer;
Call DaysBetween to obtain the difference, in days, between two TDateTime values. DaysBetween counts only whole days. Thus, DaysBetween reports the difference between Dec 31, 1999 11:59 PM and Jan 1, 2000 11:58 PM as 0 because the difference is one minute short of an entire day.
 
====
So you might say
DaysNumber := DaysBetween(MyDateSelector1.date, MyDateSelector2.date);

Of course there is an added element of logic you'll have to use.
Let's say the first day selector selects a day that falls on a Wednesday, and the second is the Friday. You will obviously only want the checkboxes from Wednesday Thursday and Friday to show.
For that you can use DayOfTheWeek from dateutils, which returns a number from 1 to 7.
Here's the syntax:
DayOfTheWeek(const AValue: TDateTime): Word;

Do you have an example on how to do it?
The daterange might start with a friday and end with a thuesday.

Sorry didnt see the disabled part.
Just change the checked property to enabled

Thank you.

uses
  DAteutils
 
Let us assume that you're using 2 tDateTimePicker's called tDateTimePicker1 and TDateTimePicker2 to let the user select a start and finish date and that they then click a button (or you do this on the OnChange event)
 
We can use Dateutils to determine the day of the week of our start date and end date.
 
we'll declare 2 variables for our start date and end date
 
var
  StartDay : integer;
  EndDay : integer;
  i : integer; //also need a counter
 
begin
 
//First of all, if the date range is larger than 7 days, then all checkboxes are checked
if DaysBetween(DatetimePicker1.Date, DateTimePicker2.Date) >= 7 then
begin
  cbMa.checked := true;
  cbDi.checked := true;
  cbWo.checked := true;
  cbDo.checked := true;
  cbVr.checked := true;
  cbZa.checked := true;
  cbZo.checked := true;
end
else
begin
  //Get the start day and end day, taking into account that the
  //stupid user may set the start date that is later than the end date.
  If DateTimePicker1.Date > DateTimerPicker2.Date then
  begin
    StartDay := DayOfTheWeek(DateTimePicker2.Date);
    EndDay := DayOfTheWeek(DateTimePicker1.Date);
  end
  else
  begin
    StartDay := DayOfTheWeek(DateTimePicker1.Date);
    EndDay := DayOfTheWeek(DateTimePicker2.Date);
  end;
  
  i := StartDay;
  while i <> EndDay do
  begin
    case i of
      1 : cbMa.checked := true;
      2 : cbDi.checked := true;
      3 : cbWo.checked := true;
      4 : cbDo.checked := true;
      5 : cbVr.checked := true;
      6 : cbZa.checked := true;
      7 : cbZo.checked := true;
   end;
    i := i + 1;
    if i > 7 then i := 1;
  end;
end;
 
//NOTE: I forget how to end case statements - I think you put an end; at the end - but anyways, you could always do 7 if ... then statements instead

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..I just love waisting my time :p

I really sorry rfwoolf.
I closed the question to early as it seems.

rfwoolf:
Your solution isn't checking the checkbox for the last day.
A small change of line 44 will do it though.
was: while i <> EndDay do
now: while i <> EndDay + 1 do

My bad. Yes that makes sense :)

Sorry rfwoolf, I started to write my answer even before your first post.
By the time I had finished and posted I saw the rest of the messages.

It's nobody's fault :)
And hey, you managed to do yours without dateuls, which is always remarkable, although I personally prefer the conciseness of the dateutils stuff.

Yeah I could probably tidy my code up a bit to be honest but it was a quick rush job :o)

Bye the way:
mikelittlewood also used DayOfWeek which I believe is also using DateUtils.
Maybe rfwoolf can sleep better now. :-)