Police said they believe at least two of the incidents are connected and that they have "persons of interest" in at least three of the cases.
Seventy-three people have been killed in Baltimore this year, compared with 74 at this time last year.
Police commanders were not made available for comment.
The first killing on Mother's Day happened shortly after noon, when 35-year-old Thomas Weddington was fatally stabbed in the 2600 block of Park Heights Terrace in Northwest Baltimore's Greenspring neighborhood. He was taken to Sinai Hospital, where he died at 1:08 p.m.
Police said the stabbing was the result of an argument and a suspect had been identified, but detectives were consulting with prosecutors to determine whether the killing was in self-defense. It's the fourth killing in the Northwest District this month and 12th of the year.
Two other shootings appeared to be connected, police said. About 3 p.m., police reported a nonfatal shooting in the 1600 block of Chilton St. in Northeast Baltimore near City College high school. A man had been shot in the torso and leg and was taken to an area hospital.
About 10:17 p.m., 19-year-old Lacy Lamb was shot in the side in the 1600 block of E. 32nd St., also near the school. Lamb, who police say lived in the block, died of his injuries.
Police said they have a suspect in custody in the nonfatal shooting and a person of interest in the killing. Lamb did not have a criminal record as an adult, and the circumstances of the shooting were not provided by police.
Detectives were investigating two other homicides:
•An officer on patrol about 4:45 p.m. heard multiple gunshots near North Loudon Avenue and West Franklin Street. He saw a large crowd dispersing and found the victim lying in the street in the 4000 block of W. Franklin St.
The 33-year-old man, later identified as Derrick Lawson of the 1700 block of N. Smallwood St., had been shot multiple times in the torso and legs. He was pronounced dead at 5:32 p.m. at Maryland Shock Trauma Center, police said.
•Homicide investigators were called to the scene of a shooting at 9:15 p.m. in the 200 block of Herring Court near Gough Street in the Perkins Homes area of Southeast Baltimore. The shooter approached a 22-year-old man and shot him multiple times in the head, according to police.
The victim, identified as Dominick Brown, was taken to Johns Hopkins Hospital, where he later died.
In addition, police were investigating the shooting of a 36-year-old woman early Saturday, who police said was struck multiple times in the upper body while sitting in her vehicle at a stoplight in the 3200 block of E. Northern Parkway. Another vehicle had pulled up and opened fire, police said.
Police initially said the woman, whom police refused to identify, was expected to survive, but on Monday a spokesman said she was in critical condition. Guglielmi said police were pulling surveillance camera footage and interviewing area businesses in hopes of retracing her steps.
Two other people were shot Friday night into Saturday morning, including a 27-year-old man in the 800 block of Bridgeview Road in Cherry Hill. According to Safe Streets Baltimore, Cherry Hill had gone 120 days without a shooting.