Mobs will spawn in blocks (even if is a for eaxample skeleton spawner during day)

PS: The command in the command block (it will maybe help):

/setblock ~ ~1 ~ mob_spawner 0 destroy {EntityId:Skeleton,SpawnRange:2,SpawnData:{DropChances:[0.1f,0.0f,0.0f,0.0f,0.0f],Equipment:[

,{},{},{},{}]}}