The earliest time to lay tefillin is when there is enough light to be able to recognise an acquaintance who is standing 4 amos away. (The same as for tsitsis – see Laws of Tsitsis #94). Those who put on tefillin in the winter as soon as dawn comes and say the beracha are doing incorrectly. (ShA1 – BeH “u’ch’sheyagia”)
Nighttime is time for tefillin. It is forbidden to lay tefillin at night only because you may fall asleep with them on and not keep a clean body. Therefore there are exceptions where you may wear them at night:
The tefillin were on you from before shkia and you have no other place to guard them. You may leave them on the entire night as long as you don’t sleep. (ShA2 – MB9)
You wake up for a trip before dawn and want to lay tefillin before your departure. You will be walking – so there’s no worry of you falling asleep. It will be difficult to don them later on. Postpone the beracha until the earliest time of daylight.( ShA 3 – MB10,11,12)
If you already daavened Maariv before tzais hakochavim and suddenly realized that you did not lay tefillin that day what should you do? Put tefillin on without a beracha until tzais. (MB 17)