Salesforce rolls out three releases a year. The schedule for the Spring '13 was published recently. When you have access to the new features is determined by which instance of Salesforce your organization is on.
You can review the schedule here:
http://trust.salesforce.com/trust/maintenance/
To find out which Salesforce instance your organization is on, login to your Salesforce account. Once successfully logged in, check out the first three characters of the url. That indicates the instance.
Once you've done that, review the schedule linked to above to see when your maintenance window will be. The good news is that Salesforce has dramatically shortened the period of downtime for their release process so your instance will only be offline for five minutes during the scheduled time.
For details on what will be included in the Spring '13 release, check out Salesforce's documentation!
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