What is the difference between 七夕节 and 情人节?
Though they are similar in significance they happen on two different dates, and therefore are not interchangeable.

七夕节 is a festival based on a very old myth. The story has many variations but simply put it tells of a young cow herder who is separated from his true love, a weaver maiden by a silver river. On the 7th day of the 7th lunar month they are rejoined when magpies form a bridge to allow the lovers be reunited.

情人节 is a festival that started around the 18th century and used the name of a christian saint for the handwritten notes of love they would exchange.

They are very different in background but have given way to the commercial ideals of consumerism. The malls seem to treat both as the same type and I often see it referred to in print as "Chinese Valetines Day"