Midavalo mentioned that there exist some free parking lots in Old Town in San Diego near the trolley that goes to San Ysidro border crossing. E.g. https://www.oldtownsandiegoguide.com/dayofthedead/parking2.html
Free and ample parking is available at the Caltrans parking lot at 4050 Taylor Street. It is free and open to the public all day both Saturday and Sunday and Monday after 5:00. It's a great place to park, as the event begins just across the street!
https://www.tripadvisor.com/ShowTopic-g60750-i41-k5242108-Free_Cheaper_Parking_in_Old_Town-San_Diego_California.html mentions other free parking spots.
For how long can one leave one's car parked in such free parking lots? 24 hours, a few days/weeks/months?