L.A.'s May 21 Election: Today is the Last Day to Register to Vote | KCET
L.A.'s May 21 Election: Today is the Last Day to Register to Vote
And by this point, the only way to really guarantee registration, is to do it online before midnight; it's handled by the California Secretary of State's office at this webpage. The other option is more cumbersome: fill out a paper form and hand deliver to L.A. County's Norwalk office by close of business.
Besides four propositions, including an advisory measure about election money, the May 21 election is the final say for a number of political offices not decided during the March 5 Primary election. KCET's Ballot Brief has been tracking (and now mapping) the direct donations to candidates in these races, all which can be seen here.