How to find the average value in a column of dates in SQL Server

If you want to average date only:

SELECT CAST(AVG(CAST(datefield AS INT)) AS DATETIME) FROM dates;

And if you want to consider time:

SELECT CAST(AVG(CAST(datefield AS FLOAT)) AS DATETIME) FROM dates;

See fiddle to test.

CONVERT(DATETIME, AVG(CONVERT(FLOAT, datefield))) as [AverageDate]