First of all, an immigrant visa is single-use; the "visa" became invalid the moment you entered the US with it. The "expiration date" of the visa is completely irrelevant now.
When you entered the US, you automatically became a US permanent resident. The "immigrant visa" automatically transformed into a proof of permanent residency (I-551) valid for one year from the date of your entry (so until Dec 24, 2016). There is a line of text on the immigrant visa that says "upon endorsement serves as temporary I-551 evidencing permanent residence for 1 year". The "endorsement" is the stamp that they put on the visa on entry. A plastic "green card" is also an I-551, and this thing that the visa turned into upon entry is equivalent to that "green card" in every way; you can use it to travel and work, etc. The fact that you don't have the "green card" yet is no matter, because you already have another type of I-551 for now. In other words, you already have in your hand what is basically a "green card" (even though it is not green) that you can use until Dec 24, 2016. Make sure to not call it a "visa", because it's not a visa anymore, and if you do, they might look at the visa expiration date and complain that it is expired.
You can re-enter the US as a permanent resident with an I-551 after an absence of less than 1 year. (And if you apply for a Re-entry Permit, you can enter during the validity of the Re-entry Permit, which is at most 2 years.) If you don't have the plastic card, you can stay out and return using just the I-551 in your passport until Dec 24, 2016. If someone sends the plastic card to you after it's received in the US, you can use that card to enter up to 1 year after you leave the US.
