On a mostly slow day of holiday trading, Zipcar Inc. investors again hit the gas. Last week, the company, which rents cars hourly or daily, reported a quarterly profit that topped estimates and raised the lower end of its annual profit forecast. On Friday, the stock had its biggest gain since April 14, 2011, the first trading day; it had fallen 55 percent this year through Thursday. “We are on track to deliver 2012 as Zipcar’s first full year of profitability,” CEO Scott Griffith said. Zipcar forecast it will earn $1 million to $4 million this year.