Add the day under Days off, in the campaign's Countdown panel. The hint under the list shows the next three cut-offs with your days off applied before you save, and the Campaigns list shows the next one and the days off it passed over, such as "Next cut-off: Tue 29 Dec, 15:00 (25 and 28 Dec off)".
A closure of several days needs a line for each day you would otherwise dispatch; weekends you never ticked need none. Tick Every year for a holiday on the same date each year, and it is skipped from then on without the campaign being opened again. A date without that tick is removed from the list the first time you save after it has passed.
A page cache does not need emptying for this: saving the campaign empties the caches the plugin knows, and every cached copy made since carries the cut-offs with your days off already left out.
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