SIDS does not occur evenly throughout the year. There are well-known patterns associated with certain seasons and times of day. There is also a suggestion in the epidemiological literature that there are spikes associated with particular dates on the calendar, including today.
Because the other two dates with spikes are New Year’s Eve and April 20 (cannabis celebration day), it may be that on dates on which some parents consume psychoactive substances heavily, those parents are less attentive to their infants, or perhaps even endanger them (e.g., rolling over on top of them during co-sleeping). But if this is the mechanism, why aren’t there spikes on Christmas and Thanksgiving, dates upon which many American consume substances in an excessive fashion?
A further mystery: Why July 5 and not the holiday itself, July 4th? Are hungover parents the problem rather than the acutely intoxicated? Are the deaths not discovered until the next day because the parents don’t notice until then, or, not recorded by understaffed hospitals until the next day?
And finally, before one engages in too much story telling and explaining, it has to be said that SIDS is a mercifully rare event and any study of it thus deals in small samples. Studies of small samples can fool us into thinking an effect is there when there isn’t (see here and here). Perhaps the three date spike of SIDS death is just chance, and we see meaning in it that doesn’t in fact exist.