php object trouble setting properties

I've been trying to use a standard object to set up and hold items against days of the week

$day = new stdClass();
$days = array('Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday');

The data is an array like this:

$activity = Array
(
    [0] => 1
    [1] => activity 1
    [2] => 1
    [3] => activity 2
    [4] => 3
    [5] => activity 5
    [6] => 4
    [7] => activity 7 ...(more entries)

First item is day of the week so Monday is 1 Tuesday is 2 etc

I want to create an object for each day. And then add the activities for the day to it. I then want to be able to iterate through each day taking out the activities one at a time and processing them. I tried this

for($x=0;$x<count($activity);$x+=2){
    $dayIs = $days[$activity[$x - 1]];
    $day->$dayIs->activity = $activity[$x+1];

This throws Cannot use object of type stdClass as array

Do I need to use a switch statement for each day and then do

$day->monday->activity = $activity[$x+1];

or is there some other way of doing this.

Also when I have the day set up how can I add multiple activities to it.

Sorry if this is an obvious thing. I can do this with arrays but I though it was time to have a go at using objects instead.