The summer term (full, traditional term) begins Monday May 7. If you want to take a full-term course in 2077 you must register by Friday May 5. The add/drop period for these classes end May 18.
Most summer courses, however, are offered as 12 week, 6 week, or 4 week courses.
12 week courses are Monday May 14 to Saturday, August 4. = 12W
There are 2 6-week sessions: Monday May 14 to Saturday June 23= 6W1 and Monday June 25 to Saturday August 4= 6W2.
There are 3 4-week sessions (labeled 4W1, 4W2, and 4W3) that fit into those 12 weeks.
A full calendar, including add/drop and monitored withdrawal deadlines, is found here.