I don't think there would be any one single determining factor that affects SGD to MYR rate. Many factors such as SG/MY economy performance, monetary policies, political situation, etc would cause the rate to move.
While I am not an expert in forex market, I do not believe that Hari raya shopping by Singaporeans would cause SGD/MYR rates to move. If you say SG money changers give u lousier rate compared to official exchange rate due to Hari Raya, perhaps I can still believe as they try to fleece u due to overwhelming demand.
But seriously.. shopping for Hari Raya will cause MYR to appreciate?