user: Question: When was the release date of the operating system with kernel version 2.6.31?
Columns: ['Version', 'Code name', 'Release date', 'Desktops', 'Servers', 'Kernel Version']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['4.10', 'Warty Warthog', '20 October 2004', 'Old version, no longer supported: 30 April 2006', 'Old version, no longer supported: 30 April 2006', '2.6.8']
SQL:

assistant: SELECT "Release date" FROM "Table" WHERE "Kernel Version" = '2.6.31';