SQL - use variable as column in SELECT statement and WHERE clause
You need to use dynamic sql to use variables as column names, try:

ALTER PROCEDURE dbo.dayplanv00 (
    @dt nvarchar(255) = '[15-04-2017]'
    ,@fg nvarchar(255)= 'XC 1 IEC'
)
AS
BEGIN
    DECLARE @sql nvarchar(200)= 'Select ' + @dt + ' as dt from RollingPlanWK15 where FNGD = ''' + @fg + ''''
    EXEC sp_executesql @sql
END


Note, are you wanting those default values in there? would make more sense (in my opinion) to have it as a required field on the execution of the proc so you don't accidentally run it with no values and presume its right