A film described as opening on Friday has usually been playing since Thursday evening. The earlier start exists because the opening weekend number is a publicity asset worth engineering.
Preview showings became part of the weekend total
Thursday evening screenings were originally treated as previews outside the official weekend, and their revenue was reported separately or not at all.
Practice shifted so that those showings are folded into the opening weekend figure, which raised reported totals without any additional audience.
Because every studio does it, the comparison across films stays fair, and the effect is a permanent upward step in what an opening looks like.
The opening figure drives everything that follows
Coverage of a release is built around the opening weekend, and that number shapes whether a film is described as a success before most people have seen it.
Theater chains use early performance to decide how many screens a film holds in its second week, so the first days affect availability directly.
A stronger reported opening therefore produces more screens, more coverage and more audience, which makes the reporting partly self-fulfilling.
Early screenings capture the committed audience
The people who attend the first possible showing are the ones who were always going to attend, and moving them earlier does not lose anybody.
It does spread demand across more showings, which reduces sellouts and lets a theater serve more customers with the same number of screens.
Fan-driven titles benefit most, because their audience is concentrated and impatient in a way a general release audience is not.
Word of mouth starts before the weekend
Reactions from Thursday screenings circulate overnight, so the film arrives at Friday with an audience verdict already forming in public.
That is an advantage for a film people like and a serious problem for one they do not, since the negative reaction now precedes the main release day.
Studios manage this with embargoes and with how early they screen for press, but they cannot control what an ordinary paying viewer posts.
The pattern is spreading further back
Some releases now add Wednesday or even earlier showings, particularly around American holidays when the working week does not constrain attendance.
Special event screenings and fan previews push the effective start earlier still, each folded into the same headline total.
What is described as an opening weekend has consequently stopped meaning a weekend, and the figure now measures something closer to a launch period.