Short notice is not a weakness if you say it first
The instinct with a late invitation is to pretend it was always this timing. Resist it. People can see the date, and an apology buried under enthusiasm reads worse than one plain sentence at the top: this is sooner than planned, it still runs, and here is what you get for an hour of your Thursday. Honest framing converts far better than manufactured calm.
What changes is the shape of the message rather than its substance. A two-week runway invites a long page of context. A two-day runway needs the same information compressed to what a person can decide on in thirty seconds: what it is about, when it starts, how long it runs, and what they can stop doing if they attend.