Section 21

Possession - Section 8 and Section 21

Questions:
1. Is the section 21 given to tenants at the same time as the new 6 month AST agreement is issued (for 4 names)?

I'm confused!

2. Or issue the section 21 just 2 months before the tenancy agreement is due to end?

3. When it is a 6 months agreement and the tenants wish to continue the tenancy, I presume the it is an ongoing tenancy, so does that mean at a time when I need to take possession of the property at a later date, then and only then I issue the section 21?

Your advice is most appreciated
Poppet