The summer or fall prior to the year of graduation is a safe bet. So, if you are recruiting students who will graduate in AY 2019-2020, you should be recruiting in the summer or fall of 2019.
Most students are actively recruiting at least one summer before they are due to graduate in the following June. The summer internship of their second to last year at MIT is an excellent opportunity to build a relationship with the students. Many companies offer their interns full time positions at the end of the summer when the students still have a year to go.
To reach more students than those you may be able to reach in your internship programs, MIT holds a career fair every fall and spring. The Fall event typically happens in the second to last week of September. Check the career fair website for exact dates.