Thanks to the Wittenberg Door for reminding us that the first edition of Heidelberg Catechism (Heidelberger Katechismus) was published on this day (19 January) in 1563. You can read  the catechism for yourself and about it here. The catechism was written to be taught to and memorized by students. It was preached to congregations in the afternoon (second) service. You can read more about that in RRC. Here’s a terrific essay by Richard Muller on why we confess the faith in confessions and catechisms. If you want to read more about the HC and other Reformed catechisms and confessions here’s a bibliography. 

update: Zac is memorizing the HC!