The crucial date is the cut-off date on which your age is calculated for eligibility in SSC exams.
It is fixed in the official notification — usually the closing date of the application or 1st August/1st January of the exam year.
You must meet the minimum and maximum age limits as on this date, not on the exam date.
Relaxation for OBC, SC, ST and other categories is also counted from the crucial date.
Always note this date from the notification before applying.