When citing a billboard, you should provide the date that the billboard was viewed, as in the below example, because such a date is the most relevant date for an ephemeral work viewed or heard in person. Ephemeral works include not only billboards but also posters, digital advertisements, broadcasts, and live performances. 

Advertisement for School of Rock: The Musical. 29 Jan. 2017, Times Square, New York City. Billboard.

The date should appear in the Publication Date element, following section 5.68 of the MLA Handbook, which states that the Publication Date element may include “the date on which a source was viewed or heard firsthand” (173).  

Note that in the example the word “Billboard” appears as a supplemental element. This supplemental element isn’t required—the Title of Source element already tells readers that the source is an advertisement—but the supplemental element may be helpful because it specifies the medium of the advertisement. For more on supplemental elements, see sections 5.105–5.119 in the MLA Handbook.

Work Cited

MLA Handbook. 9th ed., Modern Language Association of America, 2021.