The International Peace Day has been developed by UN.
This day has no significance on the islamic calender.
For muslims, every day should be one of peace and good relations towards humanity - as is reflected by the life of our prophet (sallalahu alaihi wasalam).
Muslims greet one another with: Assalamu alaikum wa rahmatullahi wa barakatuh: Peace be unto you and so may the mercy of Allah and His blessings.
In other words, islam is a religion/ way of life symbolising peace.
We should not need a specific day in the year to reflect upon this.
The fact that some mosques are getting involved in the activites of this day - is possibly, as a means of dawah (inviting others towards a religion of peace - Islam) and wanting to highlight the need for peaceful relations between others, esp. in view of the current unrest amongst the community.
Peace : )