Unit testing is done at the very begining, it is the first 
stage of testing performed by developers in their 
environment.

the most ‘micro’ scale of testing; to test particular 
functions or code modules. Typically done by the programmer 
and not by testers, as it requires detailed knowledge of 
the internal program design and code. Not always easily 
done unless the application has a well-designed 
architecture with tight code; may require developing test 
driver modules or test harnesses.  

and it has to done during the development.
each and every piece of code written by the developers for 
a project has to be unit tested.
this is the initial test that will be done from the testing 
perspective to make sure the code written by the developer 
abide by some coding standards and, code would not be 
broken in any situation

when an  development of single module of anapplication is 
completed