usually when we give name to a specific date, we use the most significant event and i think the most significant event with bitcoin is genesis block since it is the start of it all. and the block number zero (like ground zero).
so i'd say that is the only date we need.

other dates, such as the ones in my above comment (and also pizza day which he forgot) are good but not as significant.

I agree with January 9 it is close to new year which will be a good date for celebrating bitcoin day, we can also ask for a high and good year for us bitcoin users as it is the start of the year. Hope the price of bitcoin to hit the 1300$ price and be that the minimum price of bitcoin forever. Hahaha 

You should make the poll already. If a bitcoin day would be proposed, I would suggest the day the first bitcoin was mined. That is the very start of the journey of bitcoins as that was the very first bitcoin that ever existed. So January 3 would be my bet for this one, OP. Or we could also use the first ever transaction that happened which is January 12. Thanks to btvGainer for the list.

The day when the white paper has been first released, if the time is still known.

Or the time when the genesis block has been mined

It is like a philosophic question, did Bitcoin comes to live when the concept appeared or when the concept has been implemented?

Opinions vary on whether BTC should have his day or not,some may say it is too early for this but I think that this idea is not bad for discussion.In my opinion and some others members Genesis Block would be a suitable date for celebration of bitcoin day.But also good day for that can be day of first transaction.

In any case, about that day should decide bitcoin community,and since that the greatest community is here there is no better place to decide about BTC-Day.

January 3rd without any doubt. The Genesis block is the most important moment, this is when a baby get out of her mother's and start really to live. This is on this date that one's birthday is celebrate.

I will also voted for the genesis block (Jan 3) because this is truly the beginning of the Bitcoin blockchain. And it is the first mine block buy Satoshi himself.

I voted for January 9 simply because it was that day when bitcoin actually came into existence.We don't celebrate our birthdays when we as a sperm enters the egg.We celebrate it when we come into this world 
.On same logic genesis block,whitepaper etc days should not be observed as bitcoin day.

I pick January 3 : Genesis block established because in that day all bitcoin world is born.
another days could be satoshi nakamoto birthday but who knows?