"must take" is far stronger than 'are to', and suggests that there will be consequences if this is not done ( e.g. leave days not taken will be lost.
This nearly happened to me in my first job in England. I took over the running of a service at a hospital that had a long waiting list, and so worked flat out getting the waiting list down. A secretary just happened to mention that I hadn't taken my annual leave, and that if could not be carried over to the next year, but would be forfeited. I immediately went on leave, with only one day to spare.)