Q. When is it proper to use the word "curricula" (instead of "curriculum")?

Curricula” is the plural in Latin. In American English, the plural is “curriculums.” Both are correct, although in academic writing, there is a tradition of using the Latin plurals. Chicago editors follow Webster’s 11th Collegiate when forming plurals of adopted words, but please see CMOS 7.6. So depending on the type of writing, it would be a judgement call. 

Theresa