Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is a fairy tale based on Finnish mythology. It was released in 1959 as "Sampo" but several versions were made. An American version was then re-edited and released again in 1964 as The Day The Earth Froze. The title of the movie comes from late in the movie when a witch steals the sun out of the sky and locks it in her cave, causing the world to freeze. You can read a better summary of the plot at The Silver Scream.com as well as watch two versions of it, but I will give a little bit of background here in order to explain the snake scene.
A wicked witch forces her wizard minions to forge a Sampo, a magical machine that can provide its owner with anything desired - gold, silver, jewels, flour, honey, salt, etc. The head wizard tells her he can't do it so she orders him to be thrown into a snake pit to be devoured by snakes. He escapes that fate by telling her about a blacksmith who can make a Sampo. The witch then casts a spell to kidnap the blacksmith's daughter Annikki, knowing that he will have to come try to rescue her. And he does, bringing with him a young man named Lemminkainenwho has fallen in love with Annikki. The men demand that the witch give them Annikki but she tells them they must perform two simple tasks first. The first task is plowing a field that is overrun with snakes which she shows them. We see a shot of a barren rocky area with a lot of snakes lying around. Later when we see the field being plowed the snakes are rising out of the ground as if they were planted there like some bizarre crop. This is absurd, of course, there's no way snakes would all stay in a field like that - what do they eat, where do they hide, etc., but it's not impossible. I've seen similar sights at snake dens in Canada, where thousands of snakes lay around outside on the ground in the spring, which makes me wonder if they have similar dens in Finland where the movie was filmed and takes place. Maybe that's where the idea comes from. And it's not even close to being the craziest thing we see in this fairy tale.
Lemminkainen tells the witch he needs a plow and a powerful horse to do the task, but for some crazy reason she thinks he wants to feed her horse to the snakes. So the blacksmith then uses a giant forge to make a large red horse for them to use. (In the world of this story blacksmiths are almost like gods.) At first the horse is attacked by a mass of snakes crawling all over it, but it shakes them off and starts plowing the field guided by the men. Eventually the field is completely plowed and the snakes are all gone.
(Mystery Science Theater 3000 used the movie in a hilarious 1993 episode. After the field is plowed they say: "Oh great we can plant more snakes now.")
When he plows the field, a large snake attacks Lemminkainen. The wizards laugh and throw rocks at him, but he fights the snake off and continues plowing.
That's the end of the snakes but there is a lot of movie remaining. The witch's second task is for the blacksmith to make her a Sampo.
The snake field we see at first appears to be a mix of real and fake snakes then later we see a closer view of rocks and the ground draped with live snakes. It's hard to tell what kind of snakes they are but I think that the snakes we see on the horse and possibly some on the ground are harmless Grass Snakes - Natrix natrix - one of only three species of snake found in Finland.
We also see several Burmese Pythons on a rock. The large snake that Lemminkainen wrestles is also a Burmese Python. The snake fight scene is hilarious. We see him pick the snake up and wrap it around his shoulders then pretend to wrestle with it. Then they switch to a giant snake puppet, which obviously has someone's hand in its head to make it open its mouth.