Any subpoena which requires the attendance of a witness at any civil trial shall contain the following notice in a type face designed to call attention to the notice:
Contact the attorney requesting this subpoena, listed above, before the date on which you are required to be in court, if you have any question about the time or date for you to appear, or if you want to be certain that your presence in court is required.
Source:
Section 1985.2, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=CCP§ionNum=1985.2..