agoriuq
Rather than setup special cron jobs etc, just schedule a full weekly backup for Friday (or the day of your choosing) and daily incrementals on every other day including Tuesday.
This can be done very easily all from the server manager "Backup or Restore" panel.
You get a backup every day then, so your datas is "safer".
Incremental backups only use a small amount of disk space, based on the files that have changed since the last backup.
If you do not have the backup drive available on certain days (eg Mon, Wed, Thurs, Sat, Sun), then the backup will fail to run on those days. You will get an error report but that does not matter to you. The backup files (full & incremental) will still be OK to use.
Another approach is to schedule a full backup only for Friday using the default backup program.
Then install the dar2 backup contrib and configure it to run a seperate full backup on Tuesday.
That way you get exactly what you want and they would be relatively easily configured from the server manager panels. The slight complication is you are running two types of backup which will be saved in different folder names on the backup destination drive. If restoring, you will need to select the correct "default" or "contrib" restore method depending on which backup you need to restore from.