Detty December has become a major urban festival season in Lagos, Nigeria, attracting residents, diaspora visitors, tourists, artists, vendors, and hospitality operators. While the season promotes Nigerian music, fashion, food, nightlife, tourism, and creative entrepreneurship, it also raises concerns about traffic congestion, waste generation, noise disturbance, affordability, and environmental governance. This study evaluated the environmental and socio-cultural impacts of Detty December and examined support for sustainable festival policy. A quantitative descriptive survey design was adopted using data from 350 respondents who were familiar with, participated in, or were affected by Detty December activities. Data were analyzed using frequencies, percentages, odds ratios, and significance tests. Findings show that respondents strongly perceived Detty December as culturally and economically valuable, especially in promoting Nigerian culture, strengthening Lagos’s entertainment image, creating opportunities for creatives, and increasing local business income. However, traffic congestion, plastic waste, litter, air pollution, inadequate recycling facilities, noise, and diesel-generator dependence were major concerns. The study concludes that Detty December should be managed as a sustainable urban festival system through waste control, cleaner energy, traffic planning, community consultation, local vendor inclusion, and post-event reporting