In or within thirty days after June 30 or December 31 of each year except as hereinbefore provided, while a member, such member shall pay to the corporation an assessment equal to one-fourth of one percent of its share and deposit liabilities payable as shown on its financial statement as of said dates, to be used for operational purposes; provided, however, that the board may, prior to the semiannual assessment date, reduce uniformly the rate of the semiannual operational assessment or waive such assessment entirely. The assessment referred to herein may be charged to undivided earnings or operating expense.