How to have zombies spawn in day time like they do at night

Solution 1:

If you're in Creative Mode, you could use a Command Block and set this up to any type of clock:

/execute @a ~32 ~ ~32 /summon Zombie

This will spawn a Zombie 64 blocks away from all players (32 in the X axis, 32 i the Z axis.).

You could also set this up with a second one modifying the first command:

/execute @a ~4 ~ ~4 /summon Zombie

This will spawn a zombie 8 blocks away (4 in the X axis, 4 in the Z axis.).

Just set some Command Blocks up to a redstone clock and use these commands. You can modify the numbers so they spawn in different places. Just don't modify the middle number. That's how high they spawn.