Application materials must be received by April 2, 2021. That includes the letters of recommendation---speaking of which, a few words of advice. REUs of all kinds expect field-specific commentary on research readiness from recommenders, so the best advocates for a MathILy-EST applicant will be instructors who have done (or continue to do!) mathematics research themselves. College faculty are more likely than high-school teachers to have the requisite experience, with instructors at summer intensives likely somewhere in the middle.
Note about SARS-CoV-2/COVID-19: MathILy-EST will take place in 2021---it has not yet been determined whether we will meet in person or synchronously online.