How often is the lottery held in "The Lottery"?

The lottery is held every year in the summer.

Every surrounding village has a lottery.  The village in the story is so small that the lottery can be held in one day. It is held on June 27.  Every year it is held in the summer at the same time.  Tradition is very important to the people of this village. They would never consider not having the lottery, just as they do not even want to change the box that is used!

Every year, after the lottery, Mr. Summers began talking again about a new box, but every year the subject was allowed to fade off without anything's being done.

You notice that no one needs to be told what to do.  Even the children are aware of the exact procedure and help gather stones.  Since everything is done the same way every year, there is no reason to tell people what to do.  They just accept what is happening.

The lottery being held in summer just adds to the irony.  In fact, there is a mention of children being out of school for the first time that year.

The children assembled first, of course. School was recently over for the summer, and the feeling of liberty sat uneasily on most of them; they tended to gather together quietly for a while before they broke into boisterous play…

The concept of liberty is in stark contrast with what is really going on here.  The children, and their families, are in fact being held hostage by the lottery.  They are enslaved to tradition.  As we find out, the “winner” or loser, if you will, is just an innocent housewife.  No one can escape the lottery.

Jackson slowly builds suspense as it becomes clear that something is wrong in this village.  The fact that the lottery is held every year is a closely guarded tradition.  Even as barbaric and immoral as the lottery is, they are forced to continue it by peer pressure and the absence of anyone willing to stand up for what’s right.