What is the ruling on girls wearing a headband on the forehead; is it an act of the Christians?

The basic principle regarding clothing and adornment is permissibility, unless there is evidence to prove its prohibition, based on Allah's saying: "Say, 'Who has forbidden the adornment of Allah which He has produced for His servants and the good things, of provision?'" The forbidden imitation of disbelievers applies to things exclusive to them, not to things widespread among Muslims. Therefore, if hair ties are not exclusive to disbelieving women, there is no harm in wearing them.