Did Voltaire actually say "If you want to kill Christianity, you must abolish Sunday"? Recently I was listening to a defense of the observation of the Sabbath, and within it the speaker quoted Voltaire as saying something like:


  If you want to kill Christianity, you must abolish Sunday.


I've found this quote in a number of places both online and in print, such as in the International Standard Bible Encyclopedia, IV, 159, but so far I haven't found an actual reference to the writings of Voltaire.  Sadly I don't read French, so I wouldn't know how to best search for this quote in the original language either. 

Did Voltaire actually say something like this?  And if so, where/when?
There is no doubt the quote is widely attributed to him.  Several sources I checked give him credit:  


https://quotefancy.com/quote/929487/Voltaire-If-you-want-to-kill-Christianity-you-must-abolish-Sunday  
"Sunday: The Foundation of Personal and Family Peace."  https://www.crisismagazine.com/2015/sunday-the-foundation-of-personal-and-family-peace  
https://www.quotetab.com/quote/by-voltaire/if-you-want-to-kill-christianity-you-must-abolish-sunday  
Warren Wiersbe's Commentary:  Old Testament, p. 791.  https://books.google.com/books?id=GgLq2LR_cFcC&amp;pg=PA791&amp;lpg=PA791&amp;dq=%22if+you+want+to+kill+christianity...%22&amp;source=bl&amp;ots=NVNALdtNv3&amp;sig=ACfU3U2VO51Yz3rmb1HYTg1MJ_JZ1rHoxA&amp;hl=en&amp;sa=X&amp;ved=2ahUKEwjul7CzwIXpAhXHB80KHdjtAEQQ6AEwB3oECAcQAQ#v=onepage&amp;q=%22if%20you%20want%20to%20kill%20christianity...%22&amp;f=false  
https://www.quotenova.net/authors/voltaire/qddn79  [Also included are these caveats:  "There's evidence to suggest that this quote is attributed to the author, though no official sources or credible evidence could be found or has yet to be added to this page. 
We're unable to confirm that this quote is completely true and correct.
We don't suggest that you cite this quote as fact without conducting your own further research..."  
"Remembering the Lord's Day."  http://www.prca.org/resources/publications/pamphlets/item/600-remembering-the-lords-day  
https://www.quotemaster.org/q35e822b3a6bfc912e7e5eff61671d1e0  
"How Should We Keep the Sabbath?"  https://www.ewtn.com/catholicism/library/how-should-we-keep-the-sabbath-1144  


Some sources are not so sure Voltaire spoke the original: 


https://sermons.faithlife.com/sermons/114716-love-the-joy-of-life-matthew-22d-1  
The Australian Church Record, from November 24, 1927, page 11, column 1 of the pdf, uses the essence of the quote, but does not attribute it to Voltaire.  [1927-11-24.pdf, from myrrh.library.moore.edu.au]  


I did not find a reference to any of Voltaire's works that may have contained the quote.
Your safest course, it seems to me, is that, in the opinion of many, the quote is Voltaire's; however, the actual source has not yet been verified.