Backup Exec picksup the tape according to ADAMM (Advanced Device and media management) and it all depends on the OPP (Overwrite protection period) and APP (Append period) that you have set for the media set properties.
Please read this.
http://support.veritas.com/docs/192265
http://support.veritas.com/docs/237374
Incase you have a LIBRARY, then you can partition the drives for specific days tape that means if the JOB is targeted to MONDAYS partition slot then it only look in that partition and if it does not find it will ask you to insert the appropriate tape.