How can I determine the date of a snapshot in SQL Server 2008r2?
MSSnapshot_History has snapshot information for replication. 

You can join to MSSnapshot_Agents to narrow it down to your specific agent, publication, and job.

These tables are found in the [distribution] database on your distribution instance. 

Here's a basic query to get the last start time of your snapshot agents.

select a.name, max(start_time) laststart
    from MSsnapshot_agents a
        join MSsnapshot_history h 
            on a.id = h.agent_id
            where h.runstatus = 2 -- this will get you the last successful execution
        group by a.name