Source for Giving Tzedakah on the Days We Mention Yizkor :

On the holiday of Sukkot and on the last day of the yom tovs of Shavuot and Pesach outside of Israel, we read from the Torah Deuteronomy 16 : 17 which contains the phrase "Each man shall give according to his ability." Rashi says that this refers to the Korbanos (16:14), our sacrifices we used to bring to our holy Temple. Today since we cannot perform karbanos, we can fulfill the mitzvah of karbanos with our mouth.

On these 3 pilgrimage festivals in which we used to journey to our holy Temple, Bais Hamikdash, when it was standing (today we only have the wall near it), no one could show up to the Bais Hamikdash in Jerusalem empty handed since everyone would give according to their means. Since we do not have a Bais Hamikdash standing today, we are not commanded to go to Jerusalem to the holy temple today but we still have the opportunity to give. In Yizkor we say we will give charity. We do this probably to commemorate what we used to do when we had a Bais Hamikdash and also to give merit to our loved ones who have passed away. We should make sure that since we say we are going to give charity in the yizkor prayer that we do not forget after the festival to actually give. It is a mitzvah to give the Tzedakah by the very next day. It is important to make ourselves a reminder to carry out what we said. [1]

Giving Charity because of Yizkor :

In the Yizkor prayer we say that we pledge that we will give charity. If one does not intend to give charity it is very important to omit this part of the prayer. However, any amount of charity will suffice. Since it is such a great merit to your loved one, we strongly encourage you to at least give some amount of charity.

Candle Lighting for Yizkor :

It is an ancient custom to light a yizkor candle for our loved one on the evening prior to Yom Kippur, the last day of Passover and Shavuot, and on the eighth day of Sukkot, called Shemini Atzeret. A candle with a wick should be used since the wick and flame represent the body and the soul. In descending order of importance the following should be used: olive oil, a regular wax candle, paraffin candle, an electric bulb.

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[1] Pnei Baruch 39:17