On Friday, March 20th and through Sunday, each site saw a huge increase in traffic. I haven't been able to find out why this occurred; any ideas? Has anyone else seen this recently?
If nothing stands out there, then you'll have to dig a little deeper. To check for some bot/spam traffic, take a look into device, OS, geo, browser, and browser version to see if you can find a spike in a certain combination of those. (Apply secondary dimensions to dig deeper here.) Also check out your landing page(s) for this traffic too (applying an advanced segment could be useful here).
After you find out these details but still are left without an answer, you may want to try searching online for others who may have experienced similar traffic (now that you know the finer details) and you may find an answer there.
Ex. AdRoll was associated with a a big jump in traffic last year: https://www.seroundtable.com/adroll-invalid-traffic-18922.ht...
Still unexplained...Google Analytics didn't have much of an explanation for it - all direct traffic.