Application materials must be received by April 5, 2023. 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 2023, in person and with full vaccination required. If a change in pandemic conditions makes this impossible, then MathILy-EST will again take place synchronously online.