In the UK you don't celebrate Thanksgiving, is this right? Or is the holiday taking on an international status?
Generally speaking, British-born people don't celebrate Thanksgiving at all. And neither do those born in Australia.

Do you have any major British holidays we don't celebrate in the states? Boxing Day and Saint George's Day?
There are many other special days which are observed by eating particular types of food, but these are not all holidays. One that most people in the UK would know (and a favourite with children) is
Shrove Tuesday - otherwise known as Pancake Day!

Just as England celebrate St George's Day, Scotland and Wales also have special days for their own patron saints. The Welsh Have
St David's Day on March 1st, and the Scots have
St Andrews Day on November 30th. (We often hear fireworks being let off on St Andrews Day.)
Many Scots also honour the famous poet Robert Burns with a special "Burns Night" on January 25th where traditional foods such as haggis and scotch broth are eaten, accompanied by the drinking of toasts and poetry recitals. (I've never been to a Burns Night, but it sounds like fun...
apart from the haggis eating!

)
Saint George's Day begs for a custom klickie, but I'm not sure what to do with Boxing Day. That relates to alms for the poor, right?
Boxing day was traditionally the day chosen by employers to give their servants or staff small gifts of money. According to a book I have (from 1987 - therefore quite possibly out of date

), "this custom still survives when the local postmen and dustmen call to wish us "compliments of the season" in exchange for a small Christmas 'box.'" I have to say that I don't know anyone who does this where I live, but perhaps it does still happen occasionally in smaller communities.
To me, Boxing Day is just the day after Christmas, which also happens to be a public holiday.
