Just curious: What might have caused the spike in visits & views on that Sunday? A spike in the pulse of mathematics.
Screenshot from site analytics.
Per Andrew T.'s suggestion I am expanding my comment to an answer.
The date you mention was rather close to the time when Michael Atiyah announced presentation of his attempted proof of Riemann hypothesis. There were some related questions on the main. This one is now deleted: Sir Michael Atiyah's conference on the Riemann Hypothesis. Another question related to this topic is: What is the definition of the function T used in Atiyah's attempted proof of the Riemann Hypothesis? Each of those questions has over 10000 views. (For the deleted one the number of views can be checked by users with sufficient reputation who can see deleted questions. But also mere mortals can check that this question had at least 10000 views since it got a famous question badge: https://mathoverflow.net/help/badges/37/famous-question?userid=1593 - well assuming that we remember who was the asker. From SEDE we can see that the question had score 52 when it was deleted which also suggests that it was rather popular - however Data Explorer does not store number of views for deleted posts.)
There is also a discussion on meta related to these events: Is there a way to discuss the correctness of the proof of the RH by Atiyah in MO?
It is not surprising that this story caught attention of media and spread over internet. I assume that some websites also linked to the posts on MathOverflow related to this. You can find some sources linking to here simply by using Google.
Regular users probably cannot find out more about sources of the traffic for specific question or for specific date. (Not even users with access to site analytics.) Perhaps diamond-bearing users would be able to provide more detailed data about this, as they did here: Traffic from the list of hot network questions.
You can also see that the deleted question have been mentioned several times in various chatrooms on Stack Exchange network. (And a bit less also the other question.) One of those question was in the HNQ list (as some of you might remember), the other one wasn't.1 But I would expect most of the traffic related to this question to come from outside Stack Exchange network.
1There is no record of questions which appear in the hot network questions list collected by Stack Exchange. However, some time ago Glorfindel started collecting data on the HNQ questions by getting the list from the Stack Exchange site in regular intervals. He confirmed in chat that based on this data, the question 311280 was in the HNQ list and the question 311062 wasn't.