what is the best day to visit? The festival doesn’t last very long. It usually lasts for over one month. In other words, it ends in mid or late Feb. If you don’t like the crowd, just go there in early Feb before Chinese Lunar New Year( Feb 14 to Feb 19).

For details, please check this page:
https://www.travelchinaguide.com/attraction/heilongjiang/harbin/ice_snow.htm
You will find the admission fees, activities held during the festival and some other information.

If you want to visit more attractions, please have a look at this page:
https://www.travelchinaguide.com/attraction/heilongjiang/harbin/

can i go to russia after visiting harbin?

Yes, you can. You can take K19 in Harbin. It runs between Beijing and Moscow but stops in Harbin. Make sure that you have already gotten a visa to Russia.

Yes..fly to harbin from any city in china ya, there many good cheap hotel over there. Do it before march.

Than you can take a trian to Siberia cross russian.
https://www.russiantrains.com/