I guess this is a bit of good ol' wasei eigo derived from being the opposite of "off season" ⇒ ie. "on season" (where we'd more likely say "peak season").

From: http://www.andorazu.com/vocabulary.html
オンデイ （On-Day） 追加料金の発生する特定日。航空会社、券種により様々な特定日On-Dayが設定されます。

and http://www.kaoriyu.com/ask/show/20073/200732014122360083.htm... (scroll down a little past halfway).
特定日追加代金 航空会社が日本からの発着に混雑すると予想した日に追加代金を課す場合があり、これを「特定日（オンデイ）」といいます。
ゴールデンウィークやお盆、年末年始などがこの「特定日（オンデイ）」にあたります。

Maui News | Superferry service to begin Aug. 28
31, the surcharge for passengers will range from $10 to $19 depending on fare ... An adult making a round trip from Maui to Oahu on peak days (Fridays to ...
mauinews.com/news/2007/8/12/05ferry0812.html -

Taal Vista Hotel Tagaytay - Philippine Resorts - TravelPH.Com
Surcharge on Peak Days - Holy Wednesday to Easter Sunday, ... Reservation Officers have the right to change the above policies when deemed required. ...
www.travelph.com/resorts/tagaytay/taalvista/ -