§1. In the decree which accepts the libellus of the petitioner, the judge or the presiding judge must call the other parties to trial, that is, cite them to the joinder of the issue, establishing whether they must respond in writing or present themselves before the judge to come to agreement about the doubts. If from the written responses the judge perceives it necessary to convene the parties, the judge can establish that by a new decree.
§2. If the libellus is considered as accepted according to the norm of can. 1506, the decree of citation to the trial must be issued within twenty days from the request mentioned in that canon.
§3. If the litigating parties de facto present themselves before the judge to pursue the case, however, there is no need for a citation, but the notary is to note in the acts that the parties were present for the trial.
Source: Code of Canon Law (Vatican.va)