Whenever assessments are payable in installments the first installment if unpaid is delinquent at 5 p.m. on December 20th of the year in which the assessment was levied, and the second installment if unpaid is delinquent at 5 p.m. on the following June 20th. The whole amount of completion assessments if unpaid shall become delinquent on December 20th notwithstanding that assessments in the district are payable in installments. If any of the dates of delinquency in this section or in Section 26075 fall on a Saturday, Sunday or a state holiday, the assessment due on that date becomes delinquent at 5 p.m. on the next business day.