In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Imaam Muhammad Ibn Saaleh Al-Uthaymeen [rahimahullaah] said:
Celebrating Valentine’s Day is impermissible due to [several] reasons:
Firstly: It is an innovated celebration and it has no basis in the Sharee’ah.
Secondly: It calls to infatuation.
Thirdly: It is a means of preoccupying the heart with the likes of these insignificant affairs and it is in opposition to the guidance of the Salafus Saaleh [The Pious Predecessors].
Therefore, it is impermissible to initiate anything on this day related to [celebration], whether it is foodstuffs, drinks, clothing, and exchanging gifts or other than that. It is obligated on a Muslim to nobly [adhere to his (or her)] religion and should not be one who goes with the flow- following everyone who calls to something.
[Source: An excerpt from (Majmoo Fataawaa 16/199). slightly paraphrased]