Going off your answer that your dates are in the DD-MM-YYYY format, I would propose using a SQL query beforehand to correct this.
I recommend SQL as Format and FormatDate do not recognize or correctly handle 'day before month' date formats. They can output them, but they are not a recognized input.
Now the down and dirty SQL I would recommend using is:
CONVERT(DATETIME,CONVERT(DATETIME, ORDER_DATE,103),101) as ORDER_DATE
Basically what this does is take your varchar text date (Current ORDER_DATE) and convert it to a date data type (103 - which is DD-MM-YYYY). From there it then does another convert to turn the 'Day First Date' to a 'Month First Date' which is easier for most date related functions to accept as an input.
I would recommend adding a second date field here in the DE if possible. Something like
ORDER_DATE_FORMAT and have it be nullable and be an actual date type. This will then also keep your original ORDER_DATE info in the DE untocuhed.
So for instance, after you run the SQL, in your email you would have:
set @orderDate = AttributeValue("ORDER_DATE_FORMAT")
set @convertdate = FormatDate(replace(@orderDate," ",""),"l","","en-GB")
inside the email to then turn that date into the 'long' description date.
I would look at finding a way to optimize the query for converting the date - e.g. only run it on new records (e.g. those that do not have a value in
ORDER_DATE_FORMAT or similar.)