Question: Put It Down was an episode of the sitcom that began on what 2017 date?

Context paragraphs:
Put It Down (South Park): "Put It Down" is the second episode in the twenty-first season of the American animated television series "South Park".  The 279th episode of the series overall, it first aired on Comedy Central in the United States on September 20, 2017.

Emergency Medical Services Week: Emergency Medical Services Week, or EMS Week, was authorized by President Gerald Ford in 1973.  Its 2017 date is May 21-27.

Banned Books Week: Banned Books Week is an annual awareness campaign promoted by the American Library Association and Amnesty International, that celebrates the freedom to read, draws attention to banned and challenged books, and highlights persecuted individuals.  Held during the last week of September since 1982, the United States campaign "stresses the importance of ensuring the availability of those unorthodox or unpopular viewpoints to all who wish to read them" and the requirement to keep material publicly available so that people can develop their own conclusions and opinions.  The international campaign notes individuals "persecuted because of the writings that they produce, circulate or read."  Some of the events that occur during Banned Book Week are The Virtual Read-Out and The First Amendment Film Festival.  The 2017 date is September 24–30.

Abhidhamma Day: Abhidhamma Day (Burmese: အဘိဓမ္မာနေ့ ) is a Theravada Buddhist tradition celebrated primarily in Myanmar.  Abhidhamma Day celebrates Gautama Buddha's ascension into "tushita" heaven to teach his mother the "Abhidhamma".  It is celebrated on the full moon of the seventh month of the Burmese lunar year which starts in April.  The seventh month corresponds to October.  The 2017 date for this event is October 5.

Chandani Padva: Chandani Padva or Chandi Padvo is an occasion when Surtis (Gujarati people from Surat) enjoy a popular local variety of sweet Ghari, Bhushu (namkin).  The festival falls on a day after Sharad Poornima, the last full moon day in the Hindu calendar.  The 2017 date is October 6.

Tendong Lho Rumfaat: Tendong Lho Rumfaat (Prayer of the Tendong Mountain) is a festival of the Lepcha people of eastern Nepal and north-east India.  The festival occurs usually in the month of August.  According to Lepcha belief, their ancestors went atop the Tendong Mountain to escape from 40 days and 40 nights of continuous rain.  This festival commemorates that happening.  The 2017 date is August 8.

Madhu Purnima: Modhu Purnima (Bengali: মধু পূর্ণিমা , lit.  'honey full-moon') also known as Honey Full Moon Festival or Honey-offering Festival is a Buddhist festival celebrated by, the Chakma people of south-east Asia and by the Mon people of Thailand.  It occurs on the day of the full moon in the Bangla month of Bhadro (August/September).  The 2017 date is September 6.

Raja (festival): Raja (IPA: /ɾɔd͡ʒɔ/ Odia: ରଜ ) or Raja Parba (IPA: /ɾɔd͡ʒɔ pɔrbɔ/ Odia: ରଜ ପର୍ବ ) or Mithuna Sankranti is a three-day-long festival and the second day signifies beginning of the solar month of Mithuna from, which the season of rains starts.  It inaugurates and welcomes the agricultural year all over Odisha, which marks, through biological symbolism, the moistening of the sun dried soil with the first showers of the monsoon in mid June thus making it ready for productivity.  Its 2017 date is June 14-16.

South Park (season 21): The twenty-first season of the American animated sitcom "South Park" premiered on Comedy Central on September 13, 2017, and will contain ten episodes.

Daeboreum: Daeboreum (literally "Great Full Moon") is a Korean holiday that celebrates the first full moon of the new year of the lunar Korean calendar which is the Korean version of the First Full Moon Festival.  This holiday is accompanied by many traditions.  The 2017 date is February 11.

Supporting paragraphs: Put It Down (South Park) South Park (season 21)
Answer: September 13