Not everyone has unlimited Internet download. Sometimes Internet provides provide peak and off-peak hours and hardly is someone going to stay up at night to trigger their downloads.
There are many tools in Linux which allows user to set a timer to automatically start download without user intervention. Here is a simplest form of automatic off-peak download involving at, wget and shutdown command. First create a file which will contain wget and shutdown commands:
shutdown -h now
Now use at command to execute all command within mydownloads.txt file:
at -m 02:00 < mydownloads.txt
At command will automatically start download at 2AM and when downloads are finished it will shutdown your PC.
NOTE: For shutdown you need sudo or run command as root