First, take a look at your calendar or events page. Is it littered with past or expired events? Update your calendar or events to remove any irrelevant or old information, and add any upcoming events. Your calendar will feel less cluttered, by only showing current information, and it will be it easier for your users to navigate.
Once your events are modified, you will need to ensure re-directs are put into place for the former event URLs, preferably re-directing the URL to the main event or calendar page. This will prevent the original event URLs from developing into 404 errors.
Next, take time to review your site's metas. Have they been updated recently? Do they accurately reflect your website's content? If the answer to either of these questions is "no," go ahead and revise them. Ideal meta title length is about 50-60 characters, and ideal description length is 150-160 characters. Use this time to also re-visit your meta keywords. Can they be paired down? Are there more accurate terms you could be using, or are there less relevant terms you can eliminate?
While you're working with your metas, now is the time for a few more changes. Do your inside pages have individual metas? No? Consider adding them. Google is taking these into account for rankings more than they had previously. If your inside pages don't contain their own metas, they are defaulted to your site's overall metas, which in Google's eyes means duplicate content. And we all know what duplicate content can mean for Search rankings.
Like your overall metas, work to keep titles between 50-60 characters and descriptions between 150-160 characters. However, when writing these, don't worry about keywords for each page. Keywords are proving to be of less importance, especially on individual pages.
Lastly, keep an eye on your Google Search Console account, especially after making the above updates to your site. If there are any errors showing up, 404s from expired events or otherwise, get some re-directs added, and then mark as "fixed." Your "crawl errors" pages (desktop and smartphone) will be cleaned up in no time. Check out the HTML improvements tab to see how your website's content is doing. Here you will find if you have any problems with your metas, such as: duplicate meta descriptions or title tags, too long or too short descriptions or titles, and missing or non-informative title tags. You can also see if there is any non-indexable content on any of your pages under this tab.
Does your website need some Spring Cleaning of its own? We're here to help!