The Redemption Period occurs approximately 30 days after a domain has reached its expiration date. For 30 days after the expiration date, the domain will be available for renewal during a grace period. After the 30 day grace period, the domain will enter a hold period, known as the Redemption Period, for another 30 days. To recover the domain during the Redemption Period the registry requires a $150 USD fee. After the fee is paid, there is a 5 day period where the domain is Pending Delete from the registry. The Redemption Period applies to .COM, .NET and .ORG domains. Other TLDs may have different policies concerning expiration and renewal.
Here is a summary:
Redemption begins 30 days after registration on a domain expires. A domain is not available to the public until 35 days after that. Approximately 65 days total between expiration and public availability.