A full backup is a backup of all the files. It's used in contrast to the incremental backup, which is a backup of files changed since the last backup.
A common problem with full backups of networks or large file servers is that they take a long time. Backing up 300 gigabytes of data can take over half a day (over a 1gigabit ethernet network, to a SATA 3, RAID 5 NAS box).
So, full backups are typically scheduled to run over the weekend, when fewer people are using the network.
If there's too much data, a full backup may not be possible. The only solution is to split the file system into separate branches, and backup the branches on different days.
Full backups are performed in conjunction with incremental backups, usually scheduled to run once a day in the evening. A typical schedule is to perform one full backup each month, and then perform an incremental backup each evening.
Generally, it's bad to schedule full backups that fall on the 1st, last, and 15th day of the month, because those are "paydays" and it's possible that accountants may need to use the computers. (I think that mean the second and third weekends are best.) That said, backups are important enough to run even if someone's working on the weekend.