If I understand your requirement, you need long month name..
If that is write then all you need to do is change the value of variable Monthname as below.
Same with Day Name.
and then use normal function month(Date) to get the long month name.
In the following script line:
month(date(vTempDate, 'MMMM')) as MonatLong,
remove the Month() function call. Month will reset the format back to short month names. Date on its own will do the trick, like:
date(vTempDate, 'MMMM') as MonatLong,
Of course, since the underlying value will still be a date, your long month name will appear for every different date value (and every month name will appear many times in a list box). If you want to avoid that, use MonthStart to pre-format the date value, as Sunny suggested.