History Day accepts two styles for citations and bibliographies, usually referred to as Turabian and MLA.
The first of these, created by Kate L. Turabian, is typically considered a style geared for college students and can be used with all subjects. The book is A Manual for Writers of Term Papers, Theses, and Dissertations. Turabian says the list of sources at the end of your project should properly be called the "Selected Bibliography."
The second is Modern Language Association, and the book is MLA Handbook for Writers of Research Papers. MLA calls your source list "Works Cited."
Any way you slice it, you have to choose one of these styles and stick with it. It gets a little stickier, though, when you are choosing which edition of the style guide to use. MLA is in its seventh edition; Turabian is in its eighth. MLA in particular contains big changes from previous versions.
Does this mean you have to run right out and buy the most recent edition? No. But you do have to be consistent throughout. Avoid using the rules for online sources from the fifth edition and the rules for books from the seventh, for example. And yes, I know that the NHD Rule Book indicates the fifth edition of the MLA Handbook, but students can use any edition from the fifth forward (don't go backward, please, and start with the third--it's too out of date).
So you're looking for consistency at two levels: the style you choose, and the edition of the style guide.
So what's different in the most recent edition? In MLA's 7th edition, URLs are no longer required for citations of web sources. Further, after each citation you must indicate what type of sources it is by placing the medium of publication at the end of the citation; e.g., "Print," "Web," "Film," and so on. Check out Purdue's Online Writing Lab for a good overview of the changes in the 7th edition.
We'll work with judges to make sure they know about these changes and don't erroneously penalize a project. We encourage judges to focus on the quality of the sources and internal consistency within the bibliography.
And students can choose Turabian instead--historians tend to prefer it, I hear. And you can always ask us for some help, too. Good luck!