If you're referring to the time of year:
* Beginning of the year: This refers to the start of the year, typically January.
* End of the year: This refers to the end of the year, typically December.
If you're referring to a specific point in time:
* Start of the year: This refers to the beginning of the year, typically January.
* End of the year: This refers to the end of the year, typically December.
Other options:
* Beginning: This is a more general term for the start of something.
* End: This is a more general term for the finish of something.
The best antonym for "midyear" will depend on the specific context.