EF Core "Group By could not be translated and will be evaluated locally."

It's because there is no SQL query like that.

Think like SQL. If you want to get Usernames by group of Dates, you need both of those.

Basically :

await _dbContext.Holidays
    .GroupBy(h => new { h.Date, h.Username})
    .Select(g => new
        {
          g.Key.Date,
          g.Key.Username
        });

This will produce a SQL query like this.

SELECT [h].[Date],[h].[Username]
FROM [Holidays] AS [h]
GROUP BY [h].[Date],[h].[Username]

After that you can use the data to create the structure of your dictionary however you want.

The problem is that when you're trying to group in the database, you don't really have the means to materialize values inside a group. You only get to SELECT grouped columns or aggregated values (via SUM, etc.) of non-grouped columns.

For example:

SELECT [h].[Date], [h].[Username]
FROM [Holidays] AS [h]

This query would produce result set of which every row would have two columns, date and name.

Let's try grouping though:

SELECT [h].[Date], [h].[Username]
FROM [Holidays] AS [h]
GROUP BY [h.Date]

This SQL query wouldn't be evaluated at all because it's invalid from SQL server perspective. Error message would be

Column 'Holidays.Username' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

Summing all this up, you can either do what @Ercan Tirman has suggested, or, load all the usernames and dates and group them in-memory:

var dateAndUsername = await _dbContext.Holidays
    .Select(x => new {x.Date, x.Username})
    .ToArrayAsync();

Dictionary<DateTime, List<string>> grouped = dateAndUsername
    .GroupBy(x => x.Date)
    .ToDictionary(x => x.Key, x => x.Select(y => y.Username).ToList());