Bumper stickers: Avoid the 23rd, Avoid the 13th ??

I see these stickers on back of cars: Avoid the 23rd, Avoid the 13th (something like that). I assume these have some sort of police connections??

http://www.ci.san-carlos.ca.us/is/display/0,1124,deptid-18_isid-320,00.html

“Avoid the 23 is a holiday tradition in San Mateo County. All 23 law enforcement agencies join forces to crack down on the drinking driver, adding overtime hours, reassigning officers and emphasizing drinking driver enforcement with officers on regular patrol. The campaign’s methods are public education, community help and strict, quick enforcement. The Police Chiefs Association of San Mateo County sponsor the campaign. Avoid the 23 along with Avoid the 13 in Santa Clara County, Avoid the 21 in the East Bay and Avoid the Six Pack in Santa Cruz County in securing the Peninsula, East Bay and Silicon Valley against drinking drivers.”