When should I give my gift?

Probably a silly question, but as I've planned everything else about my trip so far I have nothing left to do but worry about silly questions. :)

I will be making my first trip to Japan from April 6th-20th. I have a personal guide booked for the 8th and 17th for around Tokyo. It's a volunteer guide service, and I have been emailing with my guide for several weeks now and they have been extremely helpful.

I've planned to bring a gift for my guide, as a thank you, but I'm unsure when the best time would be to give it. Would it be custom to do it first thing on the 8th, or wait until the second and last day on the 17th?

Japanese commonly give a gift at the first meeting.
After the self-introductions, "Here's a small present for you".
Something not expensive and not bulky is considered as good gift.

Wait until the last moment.
You will never know the guide is good or bad until the last moment.
I don't think you need a gift for your volunteer guides. Because they are volunteer guides.

May I ask how do you go about getting a voluntary guide?

I would give the gift at the end - if you give upon first meeting, the guide might feel obliged to give you something in return on the second day you are meeting. Also, the guide would have to carry it with him/her all day long. So I would wait till the end of the last day, and give it with a big "thank you" :)

Thank you all for your answers :)

Jessie Teoh - A friend recommended http://tokyofreeguide.com/

As for the question of knowing whether my guide is good or bad until the last moment, he has already been a HUGE help to me, and in my opinion has gone above and beyond what I would expect of a volunteer (ie unpaid and in his free time).

I don't have anything big planned, just some candies and treats that are specific to my home state as a small show of gratitude for his help and time.

My answer is solely based on my preference and has nothing to do with culture.

For me, I'd give it the last meet. I do this for a few reasons. 1. It doesn't give the other person a chance to give a gift back to me. 2. I feel like it has a bigger effect because by the end, his/her guard will be down and won't be expecting a gift as much. 3. This might be a little overboard, but I don't want to feel like the person is being nice to me because of the gift. Like they'll feel obliged to show you a good time due to the gift. I want them to be themselves.

Just my opinion.