Can I make it always night time?

Not possible without a mod. This is a good one:

http://mcpeuniverse.com/mods/always-night/

As of 0.16.0, you can change the time to night one time by

/time set night

To make it night constantly, (or if you are on lower versions then 0.16) you can use a mod:

• Simple Commands mod

  Simple Commands mod adds a few new, simple text commands. Now, among the "standard" commands will have a new command that has the effect to your character. In total there are 23 different effects, such as speed and invisibility - it is the effects of potions. With these commands, you can affect the gamegame mode, weather, time, deceleration time, set the level of experience, and more in MCPE 0.11-0.15.7

• pocketInvEditor mod (available on Google Play)

  PocketInvEditor is an inventory editor for Minecraft Pocket Edition. With this tool, you can give yourself more blocks and items without writing a single hexadecimal digit or connecting to a computer.

  • the mod also allows locking the day/night cycle

Get yourself a repeating command block and type in /time set night and let it be active, but run /gamerule commandblockoutput false first before you can activate it. If you don't run it, you will get bugged from the amount of lines outputted by the command block (unless you're okay with it).