Valentine's day, a Jahiliyya Roman festival that continued to be celebrated even after the Romans became Christians is, according to the BRITANNICA encyclopaedia, a day "lovers" express their affection with greetings & gifts.
Shaykh al-Islam Ibn Taymiyah (RH) said:
“To each among you, We have prescribed a law and a clear way”
[Surah al-Maa’idah 5:48].
“For every nation We have ordained religious ceremonies which they must follow”
[Surah al-Hajj 22:67].
“Every people has its festival & this is our festival.”
It is not permissible to give a feast or to give gifts, or to sell anything that will help them to do that for that purpose, or to allow children & others to play games as part of the festivals.
Abu Bakr came in and there were two young girls of the Ansaar with me who were singing about what
Abu Bakr said: “Musical instruments of the shaytaan in the house of the Messenger of Allaah (peace and blessings of Allaah be upon him)?!” and that was on the day of Eid.
“O Abu Bakr, every people has a festival and this is our festival.”
Imaam al-Dhahabi (RH) said:
If the Christians have a festival, and the Jews have a festival, it is only for them, so no Muslim should
Source: Tashabbuh al-Khasees bi Ahl al-Khamees, published in Majallat al-Hikmah (4/193).
When the Prophet (peace & blessings of Allaah be upon him) came to Madeenah, they had two days when they would play.
He said: “What are these two days?”
They said: “We used to play on these days during the Jaahiliyyah.”
“Allaah has given you instead of them two days that are better than them: the day of al-Adha and the day of al-Fitr.”
~Classed as saheeh by Sheikh al-Albaani (RH) in Saheeh Abi Dawood.
“Whoever imitates a people is one of them.”
Valentine’s Day is an idolatrous Christian festival, & not permissible for a Muslim who believes in Allaah and the Last Day to observe, approve of it or congratulate people on it.