Google Calendar web page has this nice mini month calendar on the left sidebar. You can just click on a day and you are navigated to the selected day on the main screen. BusyDays extension adds an extra feature in this mini-calendar. Days with appointments or events are highlighted, so you can instantly have a clear view of your busy days this month.
Download the chrome extension from Chrome web store. Once installed, go to google calendar web site, open the BusyDays extension icon, consent with providing access to your google calendar account and then select the calendars you wish to consider in BusyDays.
You may need to refresh your borwser in google calendar page and that's it! Your busy days are now highlighted in this side mini calendar. Easy?