Adding your calendar to Squarespace takes two parts: paste the Revisual loader into Code Injection once, then add a Code block to the page you want. Your events then update on your site automatically, with no changes in Squarespace.
Before you start:
You need a Core plan or higher. On the Basic plan, Code blocks accept HTML and CSS but not JavaScript, so the calendar won't show.
Publish your calendar. See How to publish your calendar and add it to your website.
In Revisual, open your calendar's Install tab and click Install. Add your website if you haven't yet, choose Squarespace and click Continue. Keep this window open: it has both snippets.
These steps are for Squarespace 7.1. On 7.0, some menus have different names. To check your version, go to Settings › Advanced.
Part 1: Add the loader with Code Injection (once)
Step 1: In Squarespace, go to Settings › Advanced › Code Injection. In newer menus, it's under Website › Pages › Custom Code.
Step 2: In Revisual, copy the first snippet. Paste it into Footer and save.
It runs on every page and doesn't change how your site looks. You only do this once per website.
Part 2: Add a Code block to the page
Step 1: Edit the page for your calendar, click + where you want it and choose Code.
Step 2: In Revisual, copy the second snippet. Paste it into the block and click Apply.
The calendar appears where the block sits. Repeat on each page that shows it.
Then click Test installation in Revisual to check it works. See How to test your calendar installation.
Add an event alert to Squarespace
An event alert has its own snippet. Copy it from the alert's Install tab and add it in Code Injection the same way as Part 1. See How to add an event alert banner to your website.
Calendar not showing?
Check your Squarespace plan: Basic doesn't run JavaScript in Code blocks.
Check that the loader is in Code Injection › Footer, and that you saved it.
Check that your website is listed under Websites in Revisual. See How to add your website so your calendar can show on it.
Check that your calendar is published.
Tip: if your main calendar holds private appointments, keep your public events in a separate calendar and show only that one. See How to organise Google Calendar for the events on your website.