A few times the new page I would be optimising for Google, I would se the new page all of the sudden, completely unexpected, on the first page in Google during the weekend. I would be happy and start the celebrations only to find out that on Monday the web site is nowhere in the SERP as it was last week.
When you are fine tuning the SEO of the web site you end up checking the ranking for the site every day. You check the ranking; you check what version of the page is in the Google cashe. You check when the latest Google XML Sitemap was submitted. You check it all against to your own log of changes of each page. And a zillion other little details in Google WebMaster Tools Console, an a whole list of third party software or services a search engine optimization professional uses.
When you check the rankings ever day you also get the feeling what will Google do and when. The more experience the more precise your predictions get. Until Google does the updates of the way it works, and it sets the quality of your prediction back somewhat.
One thing that I keep on noticing in the last two if not more years is the difference in the Google results that happen during the weekend. How does it manifest? Usually a new site pops out (out of nowhere really) somewhere in the middle of the first page of of the Google search results page. It stays there over the weekend, and on Monday morning, it is nowhere to be seen again. Sometimes it happens for a number of weekends in the row. Usually not more than 4 consecutive weekends. And what happens then? One of the Mondays Google simply does not remove the page from the first page listing. The new page that firstly appeared only during weekends, simply gets its ranking, and usually stays there where it was during the previous weekends fro months before moving up or down in the search engine results page.
You can actually use this strange feature of Google displaying different search results during weekend to check what sites are likely to get to the first page son, of who is catching up with your search engine optimization efforts.