<Li> Sean Christopher Franson as Interim Director of Investigations (seasons 4 and 6) </Li> <Li> Lorraine Bracco as Mayor Margaret Dutton . Former Public Advocate who takes over as Interim Mayor after Carter Poole resigns (season 8 - present) </Li> <Table> <Tr> <Th_colspan="2"> Season </Th> <Th_colspan="2"> Episodes </Th> <Th_colspan="2"> Originally aired </Th> <Th_colspan="2"> Nielsen ratings </Th> </Tr> <Tr> <Th> First aired </Th> <Th> Last aired </Th> <Th> Rank </Th> <Th> Viewers (in millions) </Th> </Tr> <Tr> <Td> </Td> <Td_colspan="1"> </Td> <Td_colspan="2"> 22 </Td> <Td_colspan="1"> September 24, 2010 (2010 - 09 - 24) </Td> <Td> May 13, 2011 (2011 - 05 - 13) </Td> <Td> 19 </Td> <Td> 12.58 </Td> </Tr> <Tr> <Td> </Td> <Td_colspan="1"> </Td> <Td_colspan="2"> 22 </Td> <Td_colspan="1"> September 23, 2011 (2011 - 09 - 23) </Td> <Td> May 11, 2012 (2012 - 05 - 11) </Td> <Td> 22 </Td> <Td> 12.16 </Td> </Tr> <Tr> <Td> </Td> <Td_colspan="1"> </Td> <Td_colspan="2"> 23 </Td> <Td_colspan="1"> September 28, 2012 (2012 - 09 - 28) </Td> <Td> May 10, 2013 (2013 - 05 - 10) </Td> <Td> 14 </Td> <Td> 13.17 </Td> </Tr> <Tr> <Td> </Td> <Td_colspan="1"> </Td> <Td_colspan="2"> 22 </Td> <Td_colspan="1"> September 27, 2013 (2013 - 09 - 27) </Td> <Td> May 9, 2014 (2014 - 05 - 09) </Td> <Td> 10 </Td> <Td> 13.63 </Td> </Tr> <Tr> <Td> </Td> <Td_colspan="1"> 5 </Td> <Td_colspan="2"> 22 </Td> <Td_colspan="1"> September 26, 2014 (2014 - 09 - 26) </Td> <Td> May 1, 2015 (2015 - 05 - 01) </Td> <Td> 13 </Td> <Td> 13.77 </Td> </Tr> <Tr> <Td> </Td> <Td_colspan="1"> 6 </Td> <Td_colspan="2"> 22 </Td> <Td_colspan="1"> September 25, 2015 (2015 - 09 - 25) </Td> <Td> May 6, 2016 (2016 - 05 - 06) </Td> <Td> 10 </Td> <Td> 13.07 </Td> </Tr> <Tr> <Td> </Td> <Td_colspan="1"> 7 </Td> <Td_colspan="2"> 22 </Td> <Td_colspan="1"> September 23, 2016 (2016 - 09 - 23) </Td> <Td> May 5, 2017 (2017 - 05 - 05) </Td> <Td> 8 </Td> <Td> 14.07 </Td> </Tr> <Tr> <Td> </Td> <Td_colspan="1"> 8 </Td> <Td_colspan="2"> 22 </Td> <Td_colspan="1"> September 29, 2017 (2017 - 09 - 29) </Td> <Td> TBA </Td> <Td> TBA </Td> <Td> TBA </Td> </Tr> </Table> <Tr> <Th_colspan="2"> Season </Th> <Th_colspan="2"> Episodes </Th> <Th_colspan="2"> Originally aired </Th> <Th_colspan="2"> Nielsen ratings </Th> </Tr>

When does season eight of blue bloods come out