When does an aircraft get allocated?

I was wondering how far out an aircraft gets potentially allocated for a particular flight?
For example i am on VS27 this Friday, when would an aircraft gets assigned for it.
Understanding things may change due to problems, like the 23hr delay to G-VLIP last week.
And there is not a particular flight pattern to say where its will be in the days coming up to it.
Thanks
For example i am on VS27 this Friday, when would an aircraft gets assigned for it.
Understanding things may change due to problems, like the 23hr delay to G-VLIP last week.
And there is not a particular flight pattern to say where its will be in the days coming up to it.
Thanks