Today's cactus is Gymnocalycium baldianum. This species is native to Argentina and can be quite variable in terms of flower color, ranging from deep red to white.
My plant has beautiful dark reddish-magenta flowers, and it's just gorgeous. I don't have any other cacti this color.
The flowers last for 3 to 4 days, but only open fully in bright sunlight. The close at night, then open again the next day.