When defining a date, should I use *on, upon* or *when*, or can I leave these words out?

For example, can I say:

> 
> The date the company allots the securities is known as the Despatch Date.
> 
> 
> 

or should I say: 

> 
> The date on which/upon which/when the company allots the securities is known as the Despatch Date.
> 
> 
>