Celebrations:

St. Patrick's Day celebrations around the world
Ireland:		Celebrations are often a mix of both religious services and cultural events.
United States:		Known for large parades in cities like New York and Chicago, with many businesses closing and dyeing rivers green for the occasion.
Australia, Canada, New Zealand, and South Africa:		Also celebrate with parades and events due to strong Irish immigrant communities.
Other countries:		St. Patrick's Day is celebrated in countries with Irish immigrant communities, such as Argentina, and also in countries with large Christian populations where St. Patrick is a patron saint, like Nigeria.

Holidays similar to St. Patrick's Day
St. Andrew's Day (Scotland):		Celebrated on November 30th, this day honors Scotland's patron saint. Celebrations include traditional food, music, and events similar to St. Patrick's Day, but with Scottish traditions.
St. David's Day (Wales):		Celebrated on March 1st, this day is a national day of pride for Wales, celebrating its patron saint with cultural events and traditional dress.
St. George's Day (England):		Celebrated on April 23rd, this is a national holiday in England that celebrates St. George, the patron saint of England.

The picture from Wikipedia.