One deadline looms, another has been postponed, for people interested in Mathews Market Days programs. A release from the committee notes that Saturday, June 25, is the day to submit the application and fee to take part in the poster contest. The adult fee is $15 and the student fee $5. Applications should be mailed to Mathews Market Days, P.O. Box 295, Mathews, Va. 23109-0295. A second deadline has been moved from June to July. Any organization that serves Mathews residents may apply for a Mathews Market Days grant by July 29, the release said.   Guidelines for the grant require that a person representing the organization be present at 6 p.m. Wednesday, Aug. 3, at a Market Days committee meeting in the historic courthouse. For more information, call 804-210-9169....