Bureau of the Census
The important words in the title of this organization are Bureau and Census. These are the two words that need to be capitalized. "of" and "the" are not considered important words and therefore it is not necessary to capitalize them. Often organizations use these words (of, the) in their titles. For example, the Girl Scouts of America and Big Brothers Big Sisters of America. For is also used in some other organizations like Ashoka: Innovators for the Public.