§1. A diocesan synod is to be celebrated in individual particular churches when circumstances suggest it in the judgment of the diocesan bishop after he has heard the presbyteral council.
§2. If a bishop has the care of several dioceses or has the care of one as the proper bishop but of another as administrator, he can convoke one diocesan synod for all the dioceses entrusted to him.
Source: Code of Canon Law (Vatican.va)