As the 'Grand Velsen Cathedral' does not exist, no optimal visiting time can be recommended. Churches often have specific open hours.
Since the 'Grand Velsen Cathedral' is not a real place, there is no 'best time' to visit. For actual churches in Velsen, visiting hours vary. Many historical churches are open to the public during daytime hours, typically from late morning to late afternoon, outside of service times.
To avoid disrupting religious services, it is generally best to visit outside of Sunday mornings or special religious holidays. Weekdays, particularly mid-morning or mid-afternoon, are often less crowded. For specific sites, always consult their official website or local tourist information for opening hours.