All AFS home directories (and some group areas) contain a Yesterday sub-directory. This directory contains a copy of your home directory from "Yesterday", which can be useful if you accidentally delete a file. Yesterday does not take up any of your disk quota.
The Yesterday directory is actually a by-product of our backup system. "Yesterday" usually means "from around 9pm". If the backups are running behind schedule, as can happen once a month when full backups are taken, then "Yesterday" could mean "from around 1am" or "11am" - it just depends.
This command line will tell you when your Yesterday was created (just cut and paste the whole line into a Ubuntu DICE terminal window):
/usr/bin/vos exam `fs lsmount ~/Yesterday | cut -f4 -d\' | tr -d \#` | grep Creation
To recap, "Yesterday" is usually around 9pm the day before, but for operational reasons it could be as little as "a few seconds ago".