Personally, I would express explicitly that I am participating in the general strike. I might say something like one of this:

Good morning [Boss’s Name],

I wanted to let you know that I will be unavailable on Friday, January 30, as I will be participating in a nationally organized general strike taking place that day.

This decision is personally and morally motivated and is not intended as a statement toward you, my colleagues, or the company. I remain committed to my role and responsibilities, and I will be fully available again on the next working day.

Please let me know if there is anything I should prepare or hand off in advance to ensure continuity.

Thank you,

Edit: A more subtle version without the explicit link to the strike, if you so wish.

Good morning [Boss’s Name],

I wanted to let you know that I will be unavailable on Friday, January 30.

I’ll ensure that any responsibilities are covered or handed off in advance, and I’ll be fully available again on the next working day. Please let me know if you need anything from me beforehand.

Thank you,

I like being explicit because it gives an opportunity for others to choose to join if they didn’t know about it or weren’t taking it so seriously beforehand. I try to avoid leaving the door open for their political opinions of the strike by simply not sharing my own. I state I won’t be at work, why I won’t be at work, and that is all. Not that I think it’s the right thing to do, nor that I think it’s important in any way… just that I’ll be there, not here.

If they start voicing opinions about the strike being pointless, or anything of the sort, I’ll express simply: “thanks for your perspective on the matter, but I would prefer to keep the conversation free of political bias because you never know who you might be listening and how they might feel. I just wanted to make sure you’re aware of my plans, and that it does not reflect my feelings toward you or the company in any way. Have a good day.”

Edit: fixed Thursday to Friday.