Do I need to provide a retrieval date in my reference in APA Style?
According to the American Psychological Association (2010), "do not include retrieval dates unless the source material may change over time (e.g., Wikis)" (p. 192). Wikis are designed to change over time, and so references to wikis should include retrieval dates. All other resource types (e.g., journal articles, books, web pages) can be assumed to be in their final forms and therefore references to them do not require retrieval dates. Please see "How to Cite Wikipedia in APA Style" from the APA Style Blog for an example of a reference to a wiki post.
American Psychological Association. (2010). Publication manual of the American Psychological Association (6th ed.). Washington, DC: Author.