I attempt to take on this issue through the style I love most.......that is (i.e) FAQ style.....
Question) What is St Valentine's Day?
There is an excellent writeup
about it on Wikipedia.
Question) Who likes it?
Mostly young people who like the western way of life (in which free inter-mingling of the opposite sexes is socially acceptable).
Question) Who hates it?
A few right wing nationalists oppose it bitterly. See news item
Question) Can Muslims celebrate St V..Day?
Question) Why not?
To understand this read this answer
taken from islamonline.net
Question) Is St V..... Day big business?
Read this link
to understand how Valentine's is now big bucks.
Other related links