The effectiveness of car-free day in reducing Delhi's air pollution levels is massive, provided it's not just a monthly affair.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/make-car-free-day-regular-clean-air
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/times-india-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/car-free-days
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicular-pollution
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi-pollution-control-committee-dpcc
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution-control
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/centre-science-and-environment-cse
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/who