grand festival; major festival
Refers to a large-scale festival, often with religious or traditional significance, such as a shrine's main annual celebration.
この神社では毎年秋に大祭が行われる。
A grand festival is held at this shrine every autumn.
Every autumn, a grand festival is held at this shrine.
The local people prepare early in order to take part in the grand festival.
The head priest conducted the grand festival.
祭典 is a formal, somewhat literary word for a grand ceremony or festival, while 大祭 is used specifically for major religious/traditional festivals such as shrine celebrations.
regularly held festival
Compound of 大 (large, grand) and 祭 (festival).