Rename 'mytable' as appropriate:

SELECT DateDay,

STUFF((SELECT ',' + CAST(SP2.ID AS VARCHAR(10)) FROM dbo.mytable SP2

WHERE SP2.DateDay = SP1.DateDay FOR XML Path('')), 1, 1, '') ID

FROM dbo.mytable SP1

GROUP BY SP1.DateDay

You could alternatively create a function to handle it if this is part of a larger query.