A celebration of life is planned for Saturday for Peter Sitkin, a pioneer of welfare rights and public interest lawyer, who died Dec. 29 at his home in Sonoma at age 77. Sitkin became a public interest lawyer when the practice of public interest law was relatively new. He worked for the San Francisco Neighborhood Legal Assistance Foundation, a predecessor to Bay Area Legal Aid, serving people who couldn’t afford legal assistance, in the late 1960s and early 1970s.