The candidate’s reply date is the date by which a student must say either yes or no to a single college, and beyond which no extensions are possible. 

Most colleges have agreed on May 1st as the Candidate’s Reply Date unless the student has applied and been accepted under the Early Decision plan. It is a common occurrence for seniors to have several college options toward the end of March, and they then have the month of April to make a college choice. 

"Making up one’s mind” involves sending a substantial deposit--several hundred dollars--to the college one has decided to attend.