Hmm....
sometimes it's dictated by the theatre's tradition.
I worked at a theatre in Michigan (granted, it's a community theatre) that always refers to the first sunday of tech - which is always a first tech in the afternoon, followed by dinner (provided by the board producers), and then a dress in the evening (or Act I tech in the AM, Act II tech in the evening, in the case of a super long and/or difficult show). It's called "hell sunday." That's just what it's been called there since the beginning of the theatre probably, and what everyone knows it as - all the staff, resident designers, returning directors, stage managers, actors, crew, etc. To change it up would be...like breaking tradition and ya'll know how superstitious we theatre people can get!