You may mean "When is にじゅうし(24)　used in place of にじゅうよん(24)?".
Not only 24, but also every 4 in 4, 14, 24, 34, 44, 54... is complicated in reading.
When counting 3,4,5,6, I think the majority of Japanese say san, shi, go, roku, some say san, yon, go, roku. But counting downward 6,5,4,3, all of Japanese say roku, go, yon, san.
Likewise, 23,24,25 is にじゅうさん、にじゅうし、にじゅうご. But 25,24,23 is にじゅうご、にじゅうよん、にじゅうさん.