Short answer. When you file it. A civil action is commenced by the filing of the original complaint in court — service of summons on the defendant happens afterward and is a separate step, not the trigger for when the case is considered to have started.
What the law says
A civil action is commenced by the filing of the original complaint in court. If an additional defendant is impleaded in a later pleading, the action is commenced with regard to him on the date of the filing of such later pleading, irrespective of whether the motion for its admission, if necessary, is denied by the court.
Rule 1, Section 5 — Commencement of action. Read the full provision →
Filing, not service, is the commencing act
The rule is unambiguous: a civil action is commenced by the filing of the original complaint in court. The plaintiff's own act of filing is what starts the case — not the defendant learning about it through summons, and not the court taking any further action on the complaint after it is filed with the clerk of court.
Service of summons remains an essential step for the court to acquire jurisdiction over the defendant, but it does not change when the action itself is deemed to have started.
Why this timing matters beyond a technicality
The moment of commencement matters for practical reasons, most importantly prescription: a claim generally has to be filed before the applicable prescriptive period runs out, and it is the filing date, not the date the defendant is served, that determines whether the plaintiff beat the deadline. A complaint filed on the last possible day still counts, even if service takes weeks to accomplish afterward, and a plaintiff should never delay filing while still waiting to arrange service.
A later-added defendant has their own commencement date
The rule accounts for cases where a defendant is added after the original filing: if an additional defendant is impleaded in a later pleading, the action is commenced with regard to him on the date of the filing of such later pleading. Each added defendant gets their own commencement date tied to when they were actually brought into the case, not the original filing date.
This holds even if the later pleading is initially rejected
The rule adds one more protection: this later commencement date applies irrespective of whether the motion for its admission, if necessary, is denied by the court. The filing itself fixes the commencement date for that added defendant, regardless of what happens afterward to the motion seeking to have the pleading admitted into the case before the court rules on it.