A minisymposium is a conference within a conference.
There are two kinds of minisymposia
Minisymposia accepting submissions
Minisymposia with predefined program
Minisymposia accepting submissions are tracks in the pretalx system. Simply choose the minisymposium from the "tracks" field when submitting.
By default, deadline for minisymposia is the same of JuliaCon. Some minisymposia organizers may choose to extend the deadline after this has passed. This will be communicated later.