Even with flash, .swf files don't get an association. (At least not on my box.) IE/Firefox/Your browser here will open them, however. The above shortcut does run the swf file, it runs IE, and instructs IE to open the .swf file. To change out depending on the day, you could either create an HTML file with some fancy javascript to print out the corresponding .swf, or use a program and launch the swf from there.
Code:
SYSTEMTIME stime;
char *filenames[] = {"Sunday.swf", "Monday.swf", "etc, etc" ... };
GetLocalTime(&stime);
CreateProcess("C:\\Program files\\Internet Explorer\\Iexplore.exe", filenames[stime.wDayOfWeek], NULL, NULL, FALSE, 0, NULL, "Path\\to\\swf\\files", NULL, NULL);
Didn't test that, but hopefully you get the idea. filenames should have 7 strings.
You can also make the current directory argument null and specify full filenames if that doesn't work. As always, read the manual.