Civil Code 1950.5(g)(1) allows you to mail the refund and the itemized list of deductions on the 21st day.
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The statute doesn't specify calendar days or business days, so you should assume calendar days, including holidays and weekends, and for the most protection, have the refund postmarked on or before the 21st day.
Please see the guide linked bvelow, written for non-lawyers.
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