Unfortunately, SQL Server doesn't like to have concatenated stuff passed to parameters. So, you could do something like this:
declare @strBody varchar(255);
select @strBody = 'This is to test that today is ' + CONVERT(VARCHAR(12), GETDATE(), 101) + ' and it is a good day';
EXEC msdb.dbo.sp_send_dbmail
@recipients = 'someone@yahoo.com',
@subject = 'Test',
@body = @strBody;