Its preferable that a domain name is renewed before its date of expiry. However, if you miss it, you can renew the domain name after its date of expiry as well. The outline proceduder / stages of domain after its expiry are as under:-
1. Grace Period: It starts from date of expiry and ends after 35 days of date of expiry. During this period domain can be renewed at normal rates .i.e the rates applicable to domain registration of your domain.
2. Deletion Period: After renewal period the domain passes through deletion period of 5 days. during this period it cannot be renewed.
3. Redemption Period: It starts 40 days after the date of expiry of the domain and lasts for 30 days. During this period the domain can only be renewed by its owner but registry charges very high fee for domain renewal (around $80 one time).
At termination of 70 days after the date of expiry the domain is set free by registry at its discretion and can be registered by anyone in the world on first come first server basis as a new domain.