Update (7/6/21): Within 12 hours of my writing this answer, CNN has edited their article. The original claim can be seen here. CNN now acknowledges "a 26% drop [presumably in deaths] from last year's holiday weekend." The tone of their article has slightly changed and CNN has stopped portraying the deaths over the 4th of July weekend as part of "a surge in violent crime," while continuing to report on a larger trend in violent crime.
Were at least 150 people killed by gun violence from July 2 to July 4?
At least 150 people were killed by firearms from July 2 to July 4. (Suicides and unintentional deaths are included in the 150 death figure.1)
Based on CDC firearm mortality figures for 2019, 150 deaths from firearms over a 3 day period is not unusual. Furthermore, I verified the sources for 150 deaths recorded on the Gun Violence Archive as occurring between July 2 and July 4, inclusive.
CNN writes (the sentence quoted in the question is not complete):
At least 150 people were killed by gun violence in more than 400 shootings across the country during the Fourth of July weekend as major cities nationwide confront a surge in violent crime, according to data compiled by the Gun Violence Archive.
The data, which includes the number of shooting incidents and gun violence victims nationally over a 72-hour period from Friday through Sunday, is still evolving and will be updated.
Therefore, this means July 2, July 3, and July 4.
Per the CDC, in 2019, there were 39,707 people in the US killed by firearms. That's approximately 108.8 deaths per day. If the number of deaths each day is about the same, 150 deaths over 3 days is not too unreasonable (and is actually lower than expected).
CNN cites the Gun Violence Archive. They track the number of gun deaths and include a link to a source for every incident. As of my writing this answer, the Gun Violence Archive has recorded 235 incidents resulting in a gun death from July 2 to July 4, inclusive, that have collectively resulted in a total of 258 deaths. For the sake of providing a comprehensive answer, I looked through the sources for the first 150 deaths (working backwards from the last entry on July 4) and confirmed that they are all legitimate. To be clear, some of the deaths recorded, such as this one are from accidental shootings, so not all the deaths reflected in the figure are murders.1
The first source listed, which @pinegulf has problems accessing, writes:
Two Men Dead and a Third Man in Custody in Fourth of July Shooting in
By B. Thompson
JUL 4, 2021
PONTIAC, MI – Two men were shot to death outside a Pontiac residence shortly after 1 p.m. today and the man Oakland County Sheriff’s Deputies believe is responsible for the shooting is in custody.
The victims, ages 29 and 30 and Pontiac residents, were found on the lawn of a residence in the 500 block of Valencia Drive. Deputies were sent to the residence at 1:11 p.m. on the report of multiple shots fired. The suspect, a 21-year-old Pontiac man, is being held in the Oakland County Jail.
1 I have not found a government definition of "gun violence." This document from the US Department of Justice writes:
Violence is the “intentional use of
physical force or power, threatened or actual, against oneself, another person, or against a group or
community, that either results in or has a high likelihood of resulting in injury, death, psychological
harm, maldevelopment, or deprivation” (OJJDP 2014, 14). Applying this definition to the scope of this
literature review, youth gun violence is when a gun or firearm is present in the process of a youth (ages
10–24) intentionally using force or power to threaten or harm others.
If that definition of gun violence is correct, then strictly speaking, the claim is incorrect.