Weekly changes in interest rates on money market certificates will become effective sooner, under a ruling made by the Depository Institutions Deregulations Committee yesterday.

Now the Treasury normally auctions bills on Mondays, and the money market certificates rate -- pegged to the Treasury bill rates -- becomes effective the following Thursday. Starting on April 7, the rate will become effective the day after the auction, usually Tuesday.

This closes a loophole which gave savers two days to take advantage of current rates, if they were higher than those posted for the next period. The hiatus cost thrift institiutions somewhere between $120 million and $300 million in extra interest payments during 1980.

DIDC, composed of the heads of federal financial regulatory agencies, also sent out for comment a proposal to derregulate deposit-rate ceiling by maturity. This is a phased approach to eliminating limits on certificates of deposit or to indexing them to corresponding Treasury securities, the way six-month money market certificates now are indexed.

The process would start next July, when the ceiling on certificates of five or more years' maturity would be indexed or eliminated. It would be completed by April 1, 1986, when all ceilings are removed. Interested parties have 30 days to comment.

The proposal is a compromise between starting with the eight-year certificate, advocated by the National Savings and Loan League, and the three-year certificate, advocated by Citibank. DIDC delayed any action on requests by the National Association of Mutual Savings Banks to withdraw the current cap of 12 percent on 30-month certificates and to restore a quarter-point differential between thrift institutions and commercial banks. It also deferred changing rates on NOW and passbook accounts.

The committee turned down a proposal by the American Bankers Association for a new type of certificate to compete with money market mutual funds. It also rejected permission by the Western Savings Bank to offer a floating rate certificate, also to compete with money funds. It refused to end its phase-out of finder's fees for opening certificate accounts and declined to allow penalty-free withdrawals in case of bankruptcy of the depositor.

At his suggestion, Treasury Secretary Donald Regan was elected to succeed Paul Volcker, Federal Reserve chairman. After the formal meeting, the new DIDC chairman held a press briefing. Quoting Caesar in Latin to "act as quickly as possible,' Regan said that if deregulation of interest rate ceilings can be accomplished in less than five years -- 1986 is the deadline set by Congress -- he would like to do so. Yet he declined to give a timetable.