What should I check first to explain over 300% increase in organic traffic in 1 day?

You have to first open your analytics and analyze all the landing pages. Go to Behavior then landing pages.|

When you do that check the pages that received the most hits and collect all the keywords used to find your content organically.

With that data you will need to search trends, events, terms and anything that the organic world could have done to send users to your website.

I recently experienced the same exact thing on one of my websites.

One page spiked to over 300% organic traffic, doubling the traffic of the entire site to nearly 100% increase:

I then opened up analytics, collected the keywords and then pages that received the majority of the hits. Started researching and quickly found out that it was a Christmas recipe for making some type of cultural food that sent the users to my website because I was ranking first page and third position for that term.

In January the same thing happened just last week on this same website: It started on the 7th and dropped back to normal on the 11th.

This time I haven't done the research because I already know that sometimes new trends, events, and things just happen to trigger the web to our pages. So I haven not done any research and you surely don't have to worry about. But feel free to dig deep and you will find the reason as to why that happened.

I found out it was not organic traffic at all. Our Adwords URLs lose their gclid parameter during redirect. As a (horrible) result, paid traffic goes under organic. In order to solve the problem, I exported URL list by following Acquisition > Adwords > Final URL on Google Analytics console and went over all URLs one by one.