If you want multiple categories/combinations you’ll have to edit these in your browser’s address field:

In most web browsers you can use the pipe character | instead of %7C (or), and
& instead of %26 (and) which makes it more readable.

Use the normal filter to see which abbreviation the filter has, if you for example chose World Championship you’ll see the address:

in your browsers address field so you’ll know you should use wc to identify World Championship.

And %26

You use this when multiple criteria must be met together. If you want to see the Swedish national tour events you should use this since a search on National Tour will return national tour events from all countries. To see only Swedish national tour events, enter the following in your browser’s address field:
https://www.discgolfcenter.se/kalender/?cat1=sweden%26nt

To see Swedish doubles events:
https://www.discgolfcenter.se/kalender/?cat1=sweden%26doubles

To see all PDGA major events in Europe:
https://www.discgolfcenter.se/kalender/?cat1=europe%26major

Or %7C

You’ll use this when you want to list multiple types of events, for example if you want to see all:

https://www.discgolfcenter.se/kalender/?cat1=major%7Cept%7Cet

Date search

The calendar is divided into months, so if you want to see events from May through July 2018, add:
&date1=2018-05%7C2018-06%7C2018-07
last in your adress field.

Combination

You can combine the above syntax to provide a very individual list that fits your need. Say for example that you want to see:

and you only want to see the events which match the above between april through september:
https://www.discgolfcenter.se/kalender/?cat1=ec%7Cwc%26mpo%7Cmajor%26europe%7Csweden%26nc%7Csweden%26nt%7Cept%7Cet&date1=2018-04%7C2018-05%7C2018-06%7C2018-072018-082018-09