This is pretty clearly a contract issue. Whatever your contract says applies. If you contract doesn't preclude you from performing outside the funeral home, there is no problem.
Also, many people don't understand "Conflict of Interest." It means advancing the interests of one party will damage the interests of another, and both of whom are compensating you in order to advance their interests. It does not mean doing similar work for another party.
If the mass was not to be at the funeral home in the first place, and it is not making you "unavailable" for an event at the funeral home due to the mass, then there is no damage to the funeral home's interest.
However, that is not the issue. The issue is your employment agreement / contract. If you are precluded from performing at other events in this agreement, then you are violating your agreement. Whether or not it is an actual conflict of interests is moot.