Every other date in the plan depends on this one, which is why it is read first and not last. An application deadline is fixed, and everything before it has to fit.
Different systems in different countries
The United States runs early deadlines in the autumn and regular deadlines in the winter, set by each institution. The United Kingdom runs a single central deadline through UCAS, earlier for certain courses. France runs Parcoursup on its own calendar in the winter and spring.
A student applying to more than one country is managing several calendars at once, and the earliest one governs the exam schedule.
Dates are updated each year
The pattern repeats but the exact dates move, and occasionally a system changes its structure. A calendar carried over from an older sibling's year is a good guide to the shape and an unreliable guide to the dates.
The published page for the current cycle is the source, and it is worth re reading once in the autumn.
The link to the exam calendar
Exam results take weeks to be released and further time to reach institutions. This means the last usable exam sitting is earlier than the application deadline, sometimes by more than a month.
An exam taken after that point produces a result that arrives too late, and a result that arrives too late is the same as no result at all. The last usable sitting is written down before the preparation plan is built.
The school's side of the calendar
Transcripts, school reports and teacher letters are submitted by the school, and schools set internal deadlines well before the external ones.
A student who meets the external deadline but missed the school's internal one may still have an incomplete file, and this is discovered late.
Financial aid has its own calendar
Aid applications frequently run on dates separate from admission, and in some systems they are earlier. Missing the aid deadline does not affect admission and can decide whether attending is possible.
Both calendars are collected at the start, not the admission one alone.