If you are using laravel, then $_GET is not an ideal way of testing. Also, laravel's testing setup is very nice to avoid doing things by your own.
This documentation contains examples of what you need.
Also, unit testing is about testing the expected output or may be testing the process of data transition till the output in some cases.
In your case if your test itself passes the url parameters in the GET method, then testing if it contains those parameters next to it, is not really helpful. Instead, you should assert tests for the response you expect.
Here is a very simple example :
<?php
$response = $this->json('GET', '/url',['month' => '2', 'day' => '5']);
// Check if response is successful with status code 200
$response->assertStatus(200);
// Check if response contains an expected data, for example `status` as `success`
$response->assertJson(['status' => 'success'];
If you are on laracast, this will get you started.