The most commonly used media rotation schedule is 'Grandfather-Father-Son.' This scheme uses daily (Son), weekly (Father), and monthly (Grandfather) backup sets.
This scheme begins with the daily backups. Typically, four backup media are labelled for the day of the week each backs up; for example, Monday through Thursday. Each tape is recalled for use on its labelled day. If only a one-week version history of files is maintained, then each tape is overwritten each week. In order to maintain a three-week version history of files (recommended), more tapes are required. For example this week's Monday tape will not be overwritten for three weeks. (See: When is it Safe to Overwrite? for more information).
Weekly backups follow a similar scenario. A set of up to five weekly backup media is labelled 'Week1', 'Week2', and so on. Full backups are recorded weekly, on the day that a 'Son' media is not used. Following the example above these would be 'Friday' tapes. This 'Father' media is re-used monthly.
Five weekly tapes are required in order to maintain a one-month history of files, as some months have five weeks.
The final set of three media is labelled 'Month1', 'Month2', and so on, according to which month of the quarter they will be used. This 'Grandfather' media records full backups on the last business day of each month.
If your backup plan follows a corporate fiscal calendar, then your monthly tape will take the place of the Week4 or Week5 weekly/Father tape, depending on the month.
If your backup schedule follows calendar months, then your monthly backup will vary throughout the year, replacing a daily or weekly tape. Typically, monthly tapes are overwritten quarterly or yearly (recommended), depending on version history requirements.
Each of these 'media' may be a single tape or a set of tapes, depending on the amount of data to backup and the type of backup used (incremental vs. full). Weekly and/or monthly tapes are generally considered as archive tapes. (See: Long Term Data Storage - Archiving)