If you cast a spell at the same time every year, is there a time in which a ritual like Aegis is "down"?
I had assumed not.
However the text for year duration says:
The spell lasts until Sunrise on the 4th Equinox or Solstice after its casting.
So lets assume the Winter solstice on both years fell on 21st December.
If you cast the Aegis ritual before sunrise on the 21st then it will last until sunrise on the 21st December next year. leaving a gap. between sunrise and noon.
So if it is cast before sunrise on the day of the Winter Equinox, it would remain up whilst the replacement was being cast and there wouldn't be a period when it wasn't protected, I assume that would work.
If you cast the aegis immediately before sunrise of the winter Solstice it would go through sunrise on the winter solstice, the spring equinox, summer solstice, and then come down on the sunrise of the fall equinox (FYI equinox means "equal night" and only occurs during spring and fall when days and night are of equal length. solstice means "sun still": which refers to the apparent northwards and southwards movement of the sun and occurs at the peaks- the longest and shortest days of the year. There is no winter Equinox). If you cast immediately after sunrise on the winter equinox then it will not fall until the winter equinox of the next year, at sunrise.
Durations (p. 112): Add the following paragraph immediately before "Momentary".
A magus can recast a spell with Diameter, Sun, Moon, or Year Duration at the end of its period, so that the effect is continuous, and does not briefly disappear between the two castings. The magus recasting the spell does not have to be the same person who cast the original spell, and it does not even have to be the same spell, although for a lot of spells that will not make much difference. (This is important, for example, if a covenant casts a higher-level version of Aegis of the Hearth one year.) No roll is required to achieve this, but distracting events may prevent it. The critical period is about a Diameter for Sun, Moon, or Year spells, and any distraction that interrupts the recasting of the spell is enough to guarantee at least a short break for Diameter spells. For Ritual spells, it is easy for magi to determine when they must start casting to avoid a break, as such calculations are covered by Artes Liberales and Philosophiae, but if the Ritual is interrupted and must be restarted, there will not be enough time to restart that Ritual and cast it before the older spell ends.
I realised that our covenant has a specific hook that does cause a problem here.
There is a curse that makes anybody outside of a stone building after dark assume an animal form and mindset, until Sunrise. This means we can't cast before Sunrise as most covenants would.
This has invariably allowed ghosts, (small) magical creatures and so on to sneak into the Covenant during that vulnerable time.
Any advice on how to clean out a small castle of such things?
Make a mobile stone building that allows you to go around the covenant casting the Aegis before sunrise.
Alternatively, use some Rego Vim magics to delay the onset of the Aegis, casting it before the previous sunset and releasing it at the "right" time.
Alternatively, if the covenant is smallish - the typical Boundary, no more than 100 paces in Diameter, so no more than (little over) 300 paces in circumference - how about casting a D:Ring Ward before sunset? Somewhat vulnerable, but it really has to hold for little over a night.
But if for some reason none of the above is possible, I think there are fundamentally two broad ways to go after "small calibre" supernatural intruders. The first is to a) magically enhance the sense to sniff them out b) to systematically hunt them one by one. The second is to create a T:Room Vim effect, cast repeatedly on each room and courtyard of the "castle", that either destroys them, drives them out, or captures them forcing them into a very narrow area.
My players recently decided to switch casting their Aegis to the autumnal equinox rather than the winter solstice, partly because I had carefully loaded them down with things to do near the winter solstice.