A Muslim should not celebrate this pagan festival, or join others in their celebrations of it, or attend their celebrations because it is forbidden to celebrate the festivals of non-Muslims. 

Ath-Thahabi  said: "If the Christians have a festival and the Jews have a festival, which is peculiar to them, then the Muslim should not join them in that, just as he does not join them in their religion or their direction of prayer."

This is so because the basic principles of the Salf As-salih (the pious predecessors) was Al-walaa' wal-baraa' (loyalty and friendship vs. disavowal and enmity), therefore, it is obligatory for every Muslim to follow this principle. He should love the Muslims and should not follow the non-Muslims; rather, he must be different from them. By doing so, he achieves immeasurable benefits, just as imitating them causes greater harm.
In addition to this, when the Muslims imitate non-Muslims, this makes the latter happy and fills their hearts with joy. If any Muslim girl celebrates this holiday because she sees Margaret or Hilary or whoever does so, then this undoubtedly reflects the fact that she is imitating them and approves of their behavior. But Allah Says (what means):