University closure days don't count towards the deadline for the submission of claims for extenuating circumstances. This means that your deadline to submit is extended by the number of days that the university is closed.
The deadlines for submitting an EC claim are given in 'calendar days' which covers every day of the week including Saturdays and Sundays, except for university closures days such as public holidays and Christmas closures.
Full details of term dates and university closures days can be found here.