California Code, Government Code - GOV § 84218

(a) A slate mailer organization shall file semiannual campaign reports no later than July 31 for the period ending June 30, and no later than January 31 for the period ending December 31.

(b) In addition to the semiannual reports required by subdivision (a), a slate mailer organization which produces a slate mailer supporting or opposing candidates or measures being voted on in an election shall file the reports specified in Section 84200.8 if, during the period covered by the preelection report, the slate mailer organization receives payments totaling five hundred dollars ($500) or more from any person for the support of or opposition to candidates or ballot measures in one or more slate mailers, or expends five hundred dollars ($500) or more to produce one or more slate mailers.

(c) A slate mailer organization shall file two copies of its campaign reports with the clerk of the county in which it is domiciled.  A slate mailer organization is domiciled at the address listed on its registration unless it is domiciled outside California, in which case its domicile shall be deemed to be Los Angeles County for purposes of this section.

In addition, slate mailer organizations shall file campaign reports as follows: