Hey!
I would definitely include them! The wording says to "include everything" and, even if you are not sure it is relevant, they still show your interest in the firm and that you have taken steps to engage with the legal profession.
You could just include them briefly (as there is no need to go into loads of detail), perhaps highlight anything specific that you gained from them - such as insights in certain practice areas, networking, or commercial awareness, etc. If there isn't much space, I'd focus on the other work experience entries and just briefly discuss the open days.
Whilst it might not add loads to your application on its own, it does help demonstrate consistent interest and initiative, so I don't think there is any downside to including them!
Best of luck with your application!