Akshaya Daan is he donation which becomes immortal or eternal after giving it. Means, its fruit (Phal) can be enjoyed for many many lives. The following days are good for A-Kshaya daan, that is why one must donate something on these days -

(1) Dhanu (Sagittarius), Mithun (Gemini), Meen (Pisces) and Kanya (Virgo) signs Sankraanti are called Shadasheeti days
(2) Vrish (Taurus) Vrishchik (Scorpio), Kumbh (Aquarius), and Sinh (Leo) signs Sankraanti are called Vishnupadee days

Any Punya Karm (Daan etc) done in Shadsheeti Sankraanti gives 86,000 times fruits; and in Vishnupadee Sankraanti gives 100,000 (1 Lakh) times, and on Uttaraayan and Dakshinaayan Sankraanti gives millions of times.

(3) This Punya Karm, done on Uttaraayan and Dakshinaayan Sankraanti is eternal. On Makar (Capricorn) Sankraanti, in January, one should take bath before sunrise - this gives the fruit of 10,000 cows donation. Dakshinaayan Sankraanti falls in July.

(4) Maagh Shukla Triteeyaa is called Manvantar Tithi. Who donates anything on that day, it is all eternal."
[Padm Puraan, 1/32]

(5) Vaishaakh Shukla Triteeyaa - This is called Akshaya Teej or Akshaya Triteeyaa Any Karm, bad or god, done on this day gives eternal fruits, so one should always do good Karm on this day.

(6) Solar eclipse falling on Sunday, or Lunar eclipse falling on Monday, is called Choodaamani Yog. Bathing and donating during that time give eternal fruits.

(7) Tulaa Daan and Shayyaa Daan

(8) Maagh Amaavasyaa - donate sesame seeds and offer water to Pitar