On average, what are best performing quarters of the year for the S&P 500?

I couldn't find data that included dividends, but here is the data
(within a very small margin of error) for the average value change in
the S&P for each quarter since January 1950:
Jan-Mar: -.15%
Apr-Jun: 1.34%
Jul-Sep: 1.98%
Oct-Dec: 3.81%
Total:   6.98%

I know that Vanguard has an S&P Index fund that has performed 12.1%
since inception in 1970, and that is supposed to mirror the S&P
performance.  That fund would take dividends into account.  I couldn't
get a spreadsheet of data for that fund or I would analyze it for
you... it would take more effort than I wanted to spend to get data
for every quarter since 1970 from the Vanguard website.  Here is the
performance page for that fund if you care to do the research:
https://flagship.vanguard.com/VGApp/hnw/FundsPriceHistorySec?FundId=0040&FundIntExt=INT