Admissions to U.S. Universities take place during four different semesters:
Spring – January/February
Summer – May/June
Fall – August/September
Winter – November/December

However, not all universities offer admissions to all the programs in all the four semesters. Some universities do not offer admissions for Spring, Summer and Winter semesters. Some universities do not offer admissions during Summer and Winter semesters. But, almost all universities offer admissions to almost all their graduate programs in Fall. Hence, a student should target the Fall semester for admissions as the first preference and the Spring semester for admissions as their second preference.

Deadlines for application

A student should apply for a graduate program before the specified deadline for application for a university. It should be noted that some universities may have different deadlines for applications for different graduate programs. Hence, a student should be careful in identifying the relevant deadline.

Deadlines for applications to top graduate programs start as early as one year prior to a semester start date.

Example
Semester starts: Fall season (August month of a calendar year)
Deadlines for applications start: September month of the previous calendar year

Here, it should be noted that deadlines for certain programs at certain universities are in different cycles – First deadline: September, second deadline: November, third deadline: January etc.

Also, not all universities have deadlines in the pattern above. Some universities may have a single deadline in March for a semester that starts in Fall that year.

Take an approximate deadline date for Fall season of a calendar year: December 15 of the previous calendar year.

This implies that all application materials should have reached the university by December 15.

Hence, in this example, a student should have started on the application process at least by November 15.