“庆祝” is a formal usage and this word indicates that you are happy and you would like to celebrate a festival, a memorable day or an anniversary etc.“过” is similar to“pass，go through”，for example：过桥，过河etc；“过节“中的”过“只是描述一种经历，比较平淡。 You can say"庆祝结婚纪念日"，but you'd better not say"过结婚纪念日"。