Are you trying to filter the days ? ... much better to just use your time dimension as a filter if you want to do that.
For a parameter to work, you have to create the parameter, you'll have to keep it updated (ie refresh it all the time), and then create a calculated field to use the parameter value as a filter.
A parameter is a way to drive behaviour on a viz, but once created you need to 'attach' it to your data in some way usually by a calculated field. As it stands it is a completely independent piece of information. Please have a look at some of the following articles/videos to help you understand, then post back if you're still having difficulty
Parameters (Tableau tutorial)
Wow. To be honest i think i had a case of "brainfart". Thank you for the links. I completely overlooked the use of the function "Show filter" and thought the filters were static on the left... but no!
It works now, thank you so much ^_^!
No worries at all, just mark an answer as correct to get this off the unanswered questions list.