That was modified in 2001:

The General Assembly, in resolution 55/282, of 7 September 2001, decided that, beginning in 2002, the International Day of Peace should be observed on 21 September each year.
The Assembly declared that the Day be observed as a day of global ceasefire and non-violence, an invitation to all nations and people to honour a cessation of hostilities during the Day.
http://www.un.org/events/peaceday/2004/

The International Day of Peace
1981 the United Nations General Assembly passed resolution 36/67 declaring an International Day of Peace.
In 2001, the United Nations General Assembly adopted a new resolution 55/282 declaring 21 September of each year as the International Day of Peace.
The intention of the resolution is to have the entire world observe a day of peace and nonviolence.
http://www.overcomingviolence.org/dov.nsf/0/39e6f88c1b23238dc1256ec100315c09?OpenDocument&TableRow=4.2

my information:

Plough Monday is the traditional start of the English agricultural year. Falling on the Monday between 8 January and 14 January, it is the day after Plough Sunday.
http://en.wikipedia.org/wiki/Plough_Monday

First Sunday after 6 January (”Plough Sunday”)
First Monday after Plough Sunday (”Plough Monday”)
http://www.personal.utulsa.edu/~marc-carlson/holiday/janframe.html

Plough Sunday is a traditional English celebration of the beginning of the agricultural year that has seen some revival over recent years. Plough Sunday celebrations usually involve bringing a ploughshare into a church with prayers for the blessing of the land.
It is traditionally held on the Sunday after Epiphany, the Sunday between 7 January and 13 January. Accordingly, work in the fields did not begin until the day after Plough Sunday: Plough Monday.
http://en.wikipedia.org/wiki/Plough_Sunday

First Sunday after 6 January (”Plough Sunday”)
First Monday after Plough Sunday (”Plough Monday”)
http://www.personal.utulsa.edu/~marc-carlson/holiday/janframe.html