Isn't it written mistakenly?
But the ceremonies are not spontaneous expressions of feeling; they are all customary actions to which the sentiment of obligation attaches, which it is the duty of persons to perform on certain definite occasions.
No, it's correct. The underlined part is a relative clause modifying "customary actions", meaning that it is the duty of persons to perform the customary actions on certain definite occasions
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
soheil1and 'it'?Nothing. It’s a dummy subject filling in for the extraposed subject to perform on certain definite occasions. The more basic (and less common) version of the clause is which to perform on certain definite occasions is the duty of persons.