May 27, 2020
Stanford doesn't offer "Early Decision." They have "Single Choice Early Action" which enables candidates to receive an admission verdict in December but, if admitted, to wait until May 1 to commit.
Under the rules of SCEA, candidates cannot apply elsewhere while awaiting the Stanford decision except:
Any institution, public or private, under a non-binding Rolling Admission option; Public institutions under a non-binding Early Action program;
Foreign colleges/universities on any application schedule;
Institutions whose early application deadlines are a requirement for consideration for special academic programs or scholarships only if the notification of admission occurs after January 1;
Institutions under an Interim Decision program only if the notification of admission occurs after January 1.
This information is straight from the Stanford Web site, and you can read more about their Early Action policies at:
Since you won't have a verdict from Stanford until mid-December, we urge you to ready your applications to other colleges in the event you get bad news ... or if you simply want to have a broader range of choices, even if Stanford does say "yes" in December.