where and at what time i get the quest a snake biting its tail?

You need to finish first region, then buy Hebi from the market. A snake biting it's tail is the last mission in that village Hebi.

In Hebi village, a battle against Senbokuu Eien.

This must mean you've found other accounts with that mission last finished and have yet to purchase the extra villages. Well, it's like what they said. It's the final mission of the Hebi obtained and you complete it by defeating Eien.

thanks guys its becuz my account its very very old and i already beated fisrt region without this boss so i not remembered that mission thanks all