Valentine’s Day is a jaahili Roman festival, which continued to be celebrated until after the Romans became Christian. This festival became connected with the saint known as Valentine who was sentenced to death on 14 February 270 CE. The kuffaar still celebrate this festival, during which immorality and evil are practised widely. It is not permissible for a Muslim to celebrate any of the festivals of the kuffaar, because festivals come under the heading of shar’i issues which are to be based on the sound texts. In Islam, the festivals are clearly defined and well established, and no additions or subtractions may be accepted. They are an essential part of our worship and there is no room for personal opinion. They have been prescribed for us by Allah and His Messenger (Peace and blessings be upon him). Ibn Taymiyah may Allah have mercy on him said: "Festivals are part of the laws, clear way and religious ceremonies of which Allah Says (what means): 'To each among you, We have prescribed a law and a clear way' [Noble Quran 5:48] and 'For every nation We have ordained religious ceremonies which they must follow.'" [Noble Quran 22:67] Like the Qiblah (the direction faced in prayer), prayer and fasting.

So there is no difference between joining them in their festival and joining them in their other rituals. Agreeing with the whole festival is agreeing with disbelief. Agreeing with some of their minor issues is the same as agreeing with them in some of the branches of disbelief.