How avoid weekends and vacations to "pollute" the burn down graph?

One possible solution is to have a backlog item in the sprint which contains vacations and weekends.

Example:

Assumtions for the example:

• 2 weeks long sprint (10 working days)
• Average amount of task (not user story) points completed in a sprint: 150

In the planned sprint there will be no vacations, only weekends where people will not be working on the project.

On average the team completes x points a day:
x = 150 / 10 = 15p

Based on this it is now possible to add two tasks for each weekend that is in the sprint:
Weekend 1: 30p (2 days, hence 30p)
Weekend 2: 30p

After each weekend is done the task i set as completed. The result is more predictable burndown graph.

PS: Do not add user story points for the backlog items that contains the tasks for weekends and vacations. It will effect release planing in a negative way.