Question

The Talmud in Chullin 84b cites Rabbi Yosei, who said that “with regard to a koy, one does not slaughter it on a Jewish holiday, and if one did slaughter it, one does not cover its blood.” He proves this with an a fortiori argument: “Just as circumcision, whose certainty overrides the Sabbath, whose doubt does not override a Jewish holiday; covering the blood, whose certainty does not override the Sabbath, is it not all the more so that its doubt does not override a Jewish holiday?”
From a logical standpoint this seems like a very strong a fortiori argument. There is a clear connection between overriding the Sabbath and overriding a Jewish holiday. However, the Talmud brings a refutation from Rabbi Elazar HaKappar: “What about circumcision, which does not apply on the nights of Jewish holidays—would you say the same about covering the blood, which does apply on the nights of Jewish holidays?”
This refutation is not really clear to me. After all, there is no real connection between circumcision and the nights of Jewish holidays; circumcision does not apply on a holiday night simply because it does not apply at night at all. And if you say the intention is to the very fact that the commandment of covering the blood applies all day and night, whereas circumcision applies only by day, that seems very strange. First of all, it is not at all clear that it even makes sense to say this is a leniency—that a commandment applies specifically by day—but even if we grant that, what does the fact that the commandment applies only by day have to do with the fact that its doubtful case should not override a Jewish holiday?
Does the Rabbi have an explanation of this a fortiori argument?