Comment: Postmarked by 5/21 is OK

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In reply to comment: NO, timely postmarks NOT accepted (see in situ)

Postmarked by 5/21 is OK

See the bottom of Page 13 here:
http://www.sos.ca.gov/elections/2012-elections/june-primary/...

And here (Your registration must be postmarked no later than May 21, 2012):
http://www.sos.ca.gov/elections/elections_vr.htm