My name is Lilith, I'm on the Social Care team at Shopify.
Although I don't have a reference point on the site, type of industry, marketing efforts, or other details, I can generally provide some tips to look into in a broader sense.
A large spike in traffic on a specific day could be the result of specific ad algorithms delivering ads to specific audiences on the highest active days/times. It could also be due to the timing of marketing campaigns. Among various other reasons, there may be a spike in traffic due to bot traffic purchased by a business owner. Without details and more information, we can't pinpoint that at this time. This Community tip can help you lay out some useful details so that your posts can be viewed and answered in a specific manner.
Whichever the reason may be, a few things that can help find the reason for the high bounce rate and where the traffic spike is coming from is to use an additional analytics method like Google Analytics or Lucky Orange Heatmaps. What you're looking for are patterns and trends that can help you identify the reason behind it. Google Analytics can help you track the general flow of where traffic comes from and goes after landing on the page. Lucky Orange does include some similar analytics but more importantly can give you a visual of where customers or visitors hover over on the pages they land on.
Those two methods will help you dig into what is going on with your client's business. It's also important to check with them as well to see what activity has been done to bring traffic in during the past year as this can have an impact on who's in the traffic and when the spikes occur.
Let me know how you do with the information above! I'm happy to answer any questions you think of as you work on your business.