There is an element to the citation of a regulation in the Code of Federal Regulations (C.F.R.) that seems so simple, but can in fact be different depending on what rule you choose to follow: the date of the code edition cited.
According to the Bluebook, you should cite to the most recent edition of the C.F.R. This can be tricky in and of itself. The C.F.R. has a unique publishing scheme in that it is published annually, but on a quarterly basis. Therefore, Titles 1-16 are published as of January 1; Titles 17-27 are published as of April 1; Titles 28-41 are published as of June 1; and finally, Titles 42-50 are published as of October 1. A library that still receives the paper copies might not receive them until a few months after the “publishing” date and even the online versions through govinfo.gov are not available immediately. This rolling schedule means that if you have a regulation in Title 41 and it is currently the month of February 2018, you would be citing to the 2017 C.F.R. as the date in the citation if you wanted to adhere to the requirements of the Bluebook: 41 C.F.R. § 301-10.123 (2017).
However, the Bluebook is really the province of academics. Students learn it in law school to meet the requirements that some professors may set or to work on the law review, but after law school often abandon it. So how do others cite the year of a C.F.R. provision? Sometimes, there is no year provided at all. This often happens in briefs and other filings with the court. For statutes and regulations, custom often allows you to merely cite the provision itself and forgo providing a year. This most often happens where the rule is currently in effect and has not been recently revised. A citation like this would be: 41 C.F.R. § 301-10.123.
There is yet another way to cite to the year. This method looks for when the regulation was last amended and uses that date for the year. So for the example we have been using, the citation would be: 41 C.F.R. § 301-10.123 (2009). The reason for this is, if you look at the regulation in the 2017 C.F.R. (the most recent edition available) or even the e-CFR on this date (February 23, 2018) you will see that the regulation was last amended October 27, 2009.