What is the subject of the verb 'was' before 'when' in the second clause?

"This time of year" is the subject of the verb "was."

This time of year was when the souls of the dead were said to revisit their homes.

(2) Is the expression 'the large number of' correct? in addition to 'a large number of'? I think the writer is not good at English grammar.
As America welcomed the large number of immigrants in the second half of the nineteenth century, with them came their Halloween traditions.

https://www.cambridgeschool.com/ca/the-origins-of-halloween-cat.html

"The large number," and "a large number" are grammatical phrases per se.
In the context of the sentence you refer to, the article "the" indicates a specific large number of immigrants which the writer fails to identify.

The sentence is poorly written.