With so many companies working 7 days a week, I don't think it is unreasonable that ATOS should be carrying out assessments at weekends - this allows for people who have working companions that would other wise be unable to accompany them to the assessments.
As to what to expect, this FAQ might be of help. Remember that you shold cnsder that the assessment starts as soon as you near the asessment centre and does not finish untl you are out of visual range again.
ESA Medical – What to expect