Consider this math. If according to your calendar only three weeks left, how much time for your project do you really have ? At least three days should be subtracted from 21 because of holidays. But 18 days that left value not more than 5 hour each of productive work. (Lets be realistic when it comes to creativity and intensive workout more than 5 hours per each 24 hardly possible). So 18x5=90. Ninety hours that's what you really have for you project, when according to calendar three weeks left.
We have less time, than calendars tell us.
Let's work harder!