My reply here involves only all the people that I've ever known who celebrate Easter "from the religious standpoint"
For people for whom Easter (and Good Friday) have very significant religious meaning, I don't really think people "celebrate" Good Friday. Those who are religious (and of Christian faith may go to church. (The Catholic Church has Mass on Good Friday. People are expected/required to attend.) With Lent leading up to Easter, people may continue with whatever they "gave up for Lent" (meals-wise); and if they didn't give up some things for Lent they may choose to do something along those lines at least on Good Friday.
Even people who may not "be all into" the religious roots of Easter may have some degree of respect for what is behind the observance of Good Friday for people of Christian faiths. Some people may see the day as a day when people should be "a little on the quiet side". I know someone who lived her usual day and didn't expect her kids to do much different on that day, with one exception: She'd tell her kids "not to be out running and around and screaming and laughing" on Good Friday. She didn't "weigh down" her kids with a lot of talk about the meaning of the day. She'd simply tell them, "It's Good Friday. For one day a year it won't kill anyone to find something to play that's a little quieter than running around and screaming." So I suppose one reply to the question-in-question is that some people observe (and sometimes - most times maybe for those who don't attend a church service - barely observe) the day by living it as normal, only with keeping in mind that "partying it up" type of activities aren't (or at least don't feel) appropriate to some people.