When comparing Business Calendar vs Etar, the Slant community recommends Etar for most people. In the question“What are the best calendar apps for Android?” Etar is ranked 2nd while Business Calendar is ranked 6th. The most important reason people chose Etar is:
Etar is a fork of the original AOSP calendar.
Specs
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Pros
Pro Widget support
Business Calendar offers configurable widgets in many sizes for displaying Days, Weeks, Months or Agenda views on a user's home screen.
Pro Free ad supported version available
Business Calendar comes in a free ad supported version.
Pro Smooth zoomable user interface
Business Calendar has a user interface that supports scrolling and pinch zooming.
Pro Search function
Business Calendar has a search function that allows searching the calendar for dates and events.
Pro Integrates with Google Maps
Users who input locations can use Google Maps from an event to find the directions and location.

Pro Move Calendar events from one Calendar to another
The only app that has the ability to move an event to a different calendar if it was accidentally created in the wrong calendar. This app is an essential addition to any device and for anyone with multiple calendar accounts.
Pro Events can be created using the share menu
Pro Can manipulate with multiple events at a time
The app allows selecting multiple events and move or delete them together.
Pro Open Source
Etar is a fork of the original AOSP calendar.
Pro Material design
Etar adheres to Google's Material design guidelines, i.e. it looks like a "modern Android app".
Cons
Con Poor visual design
The design of the app looks a bit dated, something out of the early 2000's. While this does not impede the usefulness of the application, it still is a sore point when compared to some of the other popular calendar apps available on Android.
Con Widgets don't show week numbers correctly on some devices
Widgets on certain devices have been showing week number incorrectly for more than 2 years. The issue hasn't been addressed and there is no sign that it will be.
Examples from S4:
Con Poor support
The developers provide virtually no help with issues.
Con No apparent way to print the agenda
An essential like printing an agenda should be very easy to do, but it's not.
