I have a MacBook & new iPhone 6. I just upgraded to OS X Yosemite, and my phone is up to date. My phone and web-based iCloud calendars are working seamlessly, but under System Preferences on my Mac I cannot get the Calendar check-mark box to stay "checked". When I check it off, and approve a "merge", it acts as though it's doing something and then suddenly it stops and my check mark disappears. I've restarted countless times, and have even deleted my iCloud account from my Mac and reloaded it altogether a few times. I'm challenged at the best of times, so perhaps this is an easy fix, but nothing logical has worked to this point which defeats the whole purpose in me carrying my Apple torch, so to speak!! Anyone else encountering such issues? I would appreciate a solution if you have, please!