It depends on the context of what "they" refers to. For example, meetings might be held weekly or monthly, while events like conferences could occur annually. Additionally, some activities, like fitness classes, may take place several times a week. Please provide more specific information for a tailored answer.
It is held every year.
it is held once in 4 years.
both are/were held every 4 years, or as the ancient Greeks called it, every olympiad
they are held every 4 years
They held with Olympics that is in every fouth year
the games where held every 4 years.
they are held every two days
The olimpic games are held every 4 years.
Sadly no.... world fairs are not held any more =(
The African cup is held every two years.
=)
Quarterly