When I enter an event in my iPhone Calendar, is does not sync to my MacBook iCal. However, if I enter an event in my MacBook iCal, it syncs immediately to the iPhone Calendar

Calendar entries do not sync between devices. Each device syncs to a calendar account server that is associated with an email account. From your description it sounds like you have 2 or more calendar databases on your Mac, but only one on your iPhone. Go to Settings/Calendar/Accounts on your phone and make sure all calendar accounts are enabled. Also go to Settings/[your name]/iCloud and make sure Calendars is turned on.

Then, on your Mac, go to Menu, System Preferences, Internet Accounts and make sure Calendars is checked for each account. Also click on the Apple ID button and make sure Calendars is checked.

What kind of calendar or calendars do you have on your iPhone and on your Mac? Are they the same?

It may have been a temporary outage on one of your mail servers.