1- 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): "O you who have believed, do not take the Jews and the Christians as allies. They are [in fact] allies of one another. And whoever is an ally to them among you - then indeed, he is [one] of them. Indeed, Allah guides not the wrongdoing people." [Quran 5:51]

One of the bad effects of imitating them is that this increases their number, as this makes it look as if they have more supporters and followers. How can it be appropriate for a Muslim who recites in every Rak'ah (prayer unit) Allah’s Words (which mean): "Guide us to the Straight Way, The way of those on whom You have bestowed Your Grace, not (the way) of those who earned Your Anger, nor of those who went astray" [Quran 1:6-7], to ask Allah for guidance to the Straight Path of the believers and to keep him away from the path of those who have earned His anger and of those who went astray, and then deliberately follows their path and imitates them?  

Statistics indicate that Valentine's Day is second only to Christmas in popularity. So, it is clear that the Feast of Love is one of the festivals of non-Muslims and that it is second only to the festival of Christmas. So, it is not permissible for Muslims to participate in such celebrations, because we are commanded to differ from them in their religion, customs and other things that exclusively belong to them, as is stated in the Quran, Sunnah (Prophet’s tradition) and scholarly consensus. 

2- A Muslim should not help the non-Muslims in their celebrations because it is one of the rituals of disbelief. Helping them and approving of what they do is helping them to manifest disbelief and make it prevail, and approving of it. The Islamic religion does not allow a Muslim to approve of disbelief or help others to manifest it and make it prevail.

Hence, Ibn Taymiyyah  said: "It is not permissible for the Muslims to imitate them in anything that is specifically part of their festivals, whether it is food, clothing, bathing, lighting fires, or changing a custom of daily life or worship."

3- A Muslim should not support other Muslims who celebrate this pagan festival. Rather, it is obligatory to denounce them, because for the Muslims to celebrate the festivals of non-Muslims is an evil action which must be condemned. Ibn Taymiyah  said: "Just as we should not imitate them in their festivals, we should not help a Muslim to do that as well; rather, he should be told not to do that."