The Islamabad authorities have announced a Public holiday on Saturday and Monday (18th and 20th December) on account of the Organization of Islamic Cooperation (OIC) Foreign Ministers Conference.

Earlier, it was announced in a notification that there will be a public holiday in Islamabad on 20 December 2021 (Monday) due to the security of various delegations of the 17th Extra-Ordinary OIC Session who will be departing to their country.
However, the Islamabad authorities have announced a Public Holiday on Saturday and Monday (18 and 20 December 2021) within the revenue limits of the Islamabad Capital Territory.
Furthermore, the hiking trails and Metro Bus Stations from Shaheed-e-Millat Secretariat to Pak Secretariat shall remain closed from 17th December to 20th December 2021.
Notification for local holiday on 18th and 20th December pic.twitter.com/RZBsX5KnAY
— Office of Deputy Commissioner Islamabad (@dcislamabad) December 17, 2021
Here is the copy of the notification:

It is pertinent to mention that mobile phone services will not be suspended in Islamabad as reported previously. The Interior Minister Sheikh Rashid and Deputy Commissioner Islamabad Hamza Shafqaat.
Read more: Mobile Phone Services Not Being Suspended in Islamabad.