When are new portals added?

Solution 1:

Since 2013-02-22 new portals are added on Friday around 2AM UTC every 1-3 weeks.
I think they add new portals once in a few weeks at no specific time.

First time they added new portals was ~2012-12-18, Source
Second time it was ~2012-12-26, source
Third time ~2013-01-25, source1 source2
Fourth time ~2013-01-30, source1 source2
Fifth time ~2013-02-22, Source
Sixth time ~2013-03-01, Source1 Source2
Seventh time ~2013-03-22, Source
Eighth time ~2013-03-29

Maybe if we see more new portal aditions around Tuesday-Wednesday we can make assumtion that new portals appear in a middle of a week.
[edit]:It seems that portals are added mostly in a middle of the week, Tuesday-Friday.

Althought Niantic Labs employee said recently that they have a new system for adding portals and its going to be much faster. So maybe things will not be the same after 2013-02-17 and maybe new portals be on Fridays.

Btw it was only 3 times they were added after 2013-02-22, so my answer is not 100% accurate.