Open source projects depend on newcomers who stay, yet most leave after a single contribution. Contribution events such as Google Summer of Code, LFX Mentorship, Hacktoberfest, and 24~Pull Requests attract thousands of newcomers each year, but whether they produce lasting contributors remains unclear. We conduct the first matched-cohort study comparing 2,001 event-based and 2,001 organic contributors across 330 projects. Our results reveal three key findings. First, event contributors have significantly higher odds of becoming core contributors (12.1% vs. 9.6%, p < 0.001, OR = 1.31) and stay twice as long (median 8.2 vs. 4.8 months). Second, each entry mechanism produces a fundamentally different engagement rhythm: 62.3% of mentorship contributors sustain Steady weekly activity across their first 12 weeks, whereas 82.3% of non-mentorship and 77.6% of organic contributors exhibit Front-Loading—an initial burst that fades quickly (p < 0.001). Third, Steady engagement predicts significantly longer retention regardless of group (median 13 vs. 8 months for Front-Loading), yet mentorship contributors who lose their program scaffolding show shorter retention than self-sustained non-mentorship contributors, revealing a mentor-dependency effect. A newcomer’s first 12 weeks are strongly indicative of their long-term trajectory.