A sitting is defined as attendance at an Exam Centre for one day.
That might be the CAA's gold-plated interpretation, but it is NOT the EASA definition!
AMC1 FCL025 para e):
(e) ‘Sitting’: a period of time established by the competent authority within which a candidate can take an examination. This period should not exceed 10 consecutive days. Only one attempt at each examination paper is allowed in one sitting.