You can execute a task at the time you want it to launch by using at. Let’s say you want mplayer start playing a movie at 12:42. Do a

at 1242

A command prompt will appear where you can type the command you cant to execute at 12:42

mplayer movie.avi

Next, press Enter then press Ctrl+D to exit the command line. The job will run in the background and will execute the command at the time requested.
You can always view a list of scheduled commands with at -l or remove a task with atrm 1 (where 1 is the job ID given by at -l).