Essentially, Christmas is not a 'western' festival, as you call it. It is a festival in memory of the birth of Jesus Christ, the Messiah of the entire world, not just about west or east. Christianity is global, which makes its celebration universal. Although much of the reason why it's so festive today is the shopping atmosphere. Still, only because the day is important enough that businessmen can make use of it to earn good money.
Although the seond 'western' festival you mentioned is not about religion per se. Interestingly and coincidentally here, Valentine is a Christian saint, isn't he? Lol!