This report summarizes the findings from cognitive testing of modified mail materials for use in the 2019 American Community Survey (ACS) Due Date Test Experiment. In this report, we qualitatively assess experimental features of the fifth mailing to ensure they successfully communicate desired messages to respondents. One feature tested was the addition of a response “due date” in the letter with a corresponding message explaining that an interviewer may be coming to a respondent’s home if a response is not received by that date. Three variations of this message (positive, negative, and neutral) that varied in the language used to describe receiving a visit from an interviewer were tested. We also tested the addition of a due date on the envelope for the fifth mailing. Cognitive testing assessed participant comprehension of this new language. In general, this language was well understood, though some participants had difficult with the negative version of the message. Recommendations were made to improve respondent comprehension of the mailing prior to the field experiment.