Hey there! Several people above have mentioned this and I probably am repeating some of their points, but it is due to timetable clashes and the popularity of the subjects. Content-heavy subjects always seem to be together, and for someone who did pretty much all content heavy subjects (Bio, Chem, Econ, Maths, English), it was a horrible experience to have 2 exams in the same day, let alone one in the afternoon and one in the morning after.
What it ultimately boils down to is your study schedule and your stress levels. Make sure that you have regular breaks (if you prefer them), that your study time is actually study time (no distractions, use a variety of methods to drill the content in your head), exercise (helps with stress levels due to endorphins pumping through your blood), healthy eating, etc. There are a variety of dedicated articles here as well which I'll link when I find them. Consistent study throughout the year, and taking a relaxed approach the week before (not cramming, though that works for some students, it doesn't for others due to increased stress) but taking it easy in terms of studying.
Hope this helps!