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

Good morning [Boss’s Name],

I wanted to let you know that I will be unavailable on Thursday, 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,

Good morning [Boss’s Name],

I wanted to let you know that I will be unavailable on Thursday, 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,