Calendar data suddenly disappeared in macOS Sequoia

Calendar data is typically stored on a server (e.g. Apple iCloud, Google Gmail, Microsoft) that is then connected to by each device that uses the calendar. In that way, all devices see and can use exactly the same calendar information. To get your iPhone and Mac to display the same calendar, you simply need to make sure that the same email/contacts/calendar accounts are added to both.

Add email accounts in Mail on Mac - Apple Support

Add an email account to your iPhone, iPad, or iPod touch - Apple Support

After reading a slew of articles & sad posts & head-scratching replies, it's clear I'm staying w/ Sonoma until the multitude of harsh bugs & glitches are worked out of Sequoia. Right now (in mid-January 2025), 15.+ still looks like a complete mess of an OS.

I'm sorry that you feel that way because I have had an excellent experience with Sequoia, at home and at work, on a new Mac Mini M4, a Mac Book Air M1, and a MacBook Air M2, and both a new install and upgrade. I have honestly not had a single problem with the OS in either my home environment or an enterprise environment.

New OS versions absolutely bring the potential for new bugs and changes that are unpopular, but most significant issues are worked out within the initial minor update. I will remind you to put what you read in this community and elsewhere online in perspective. The people that come here and to other online forums and report problems are the relatively tiny fraction of Apple product users that are experiencing some kind of issue. For every one person reporting a problem with an Apple product, there are millions that are not.

If you are concerned about specific perceived "harsh bugs & glitches" in Sequoia, start a new post of your own and ask the knowledgeable community members here for information and if your concern is justified. Just because others are discussing a problem on their particular hardware, software, and working environment, does not mean that you will experience the same issue in your situation.