When is the best time to water your fig tree in pot or ground?
And how often?

Also, is Collected rainwater best for watering , or it doesn't matter?

Thank you in advance

Selfwatering pots don't care :)
For the in-ground plants and regular pots I was suggested to water in the early morning since the evening watering keeps the plants in a soggy medium for too long and as such prone to molding (esp. if the night temp is high).
As far as the rainwater goes, from what I have read depends on your tap water quality.
Some municipal water has bad pH/salt content/balance.
In general, the rainwater seems to be better if you have it.

in the morning is the best time. if water gets on the leaves, it will have time to dry before the nightfall. lessens the chance of any issues related to too much moisture. 

We have always run the drip system at 5 AM.  In the heat of summer, we run it late afternoon too, especially for grapevines that crave water.

Suzi

Many factors influence this.

Drip
Overhead
Weather
Soil type:  Clay, Sandy... etc. 
Container
In Ground
Sun or Shade or Partial

See?  Everybody's figs get watered differently.  It's best to get a moisture meter, and only water when it is showing barely moist.  We water our figs morning and night, every 3rd day.  On exceptionally hot weeks, we increase that, BUT we live in extreme heat!!

Hope that helps.

Suzi

I don't water my in ground trees, I just let Mother Nature do that. The potted trees usually need it every day and when it gets really hot sometimes they get watered two times a day, depending on pot size. The smaller the container the faster they dry out. Like Suzi said, many factors will determine how often they will need to be watered. I'm sure you will find your own watering regimen this summer.

I agree with Pete & Suzi. As a general principle, morning is the preferred time to water plants. The stomatal openings on the leaves are open during the day to allow for gas exchange (CO2 in, O2 out). Water vapor is lost when the stomata are open. The stomata close at night since photosynthesis stops with the absence of light. Which means there is no need for gas exchange and no water loss. Your trees are doing most of their water use during the day. A reasonably good book I use that is pretty easy to follow and answers a lot of this sort of growing question is:
http://www.amazon.com/Botany-Gardeners-Edition-Brian-Capon/dp/160469095X/ref=sr_1_1?ie=UTF8&qid=1364883829&sr=8-1&keywords=botany+for+gardeners

As to all the factors to consider, I think Suzi's list is pretty complete.

Good luck with your trees!

only water as needed.. but that means sometimes i have to water twice during the summer. cooler months, maybe once or twice a week. i don't use heavy things in my soil mix like sand or pebbles. so when the soil is closed to being dry, i know it's time to water. i tilt my containers almost everyday to feel how they are doing as far as weight is concerned. i know when they are light enough for watering. also, the mix i have will not take in water if they are too dry, so i have to water before they are completely dry.

there are lot of factors involved in when you will need to water your trees. it's hard to say what would be the ideal interval for your area.

usually, bigger container treees will tolerate over/under watering much better than the smaller plants.

Figs are not keen on too much water. They happy with just a little, be they on pots or 'in ground'. After living with them for a while, arrived to a point where I found that their more or less need for a 'drink' is given by their leaves' attitude. Trees with the correct dose of water (better,  soil with ideal humidity levels) show flat leaves, slightly rigid, no curls on the edges, sometimes sticking upwards,  etc.. When they start loosing that flatness/rigidity, commencing to 'look downwards', then it is time for a drink.
Our daily watch and experience tell us ahead of time when a fig needs water
Bigger or smaller trees in ground or in pots with a great mass of leaves need closer attention as they require more frequent irrigation.