The Seraphim are always serving God; they never stop.  The Seraphim's bodies are covered with eyes on all sides, so they can constantly watch God in action.  St. John The Apostle described a vision of a Seraphim in
Revelation, Chapter 4 verse 8 in the Bible
Vision of Heavenly Worship
Each one of the four living creatures had six wings, and they were covered with eyes, inside and out. Day and night they never stop saying:

"Holy, holy, holy is the Lord God      Almighty,    who was, and who is, and who is to      come".

Three True Accounts
Seraphim Angels On Earth

Seraphim spend most of their time directly in God's presence in Heaven.
They sometimes do visit Earth on special, God-given missions.
The first two known accounts of a Seraphim in heavenly form interacting with two different souls on earth.  The third known account a procession of the elect the Seraphim, with God on His Earth.

Two of the three accounts have full dates that are highlighted in bold font.

First
St. Bonaventure from the Franciscan Order composed the Life of St. Francis of Assisi.  He described in detailed St. Francis receiving his Stigmata from a Seraphim Angel on September 14, 1224.

This took place at La Verna, Italy on a mountainside, with other monks nearby, when St. Francis was praying focusing especially on the crucifixion because that day was the Feast of the Exultation of the Holy Cross.  While praying St. Francis Assisi saw one of the highest-ranking angels in creation approaching him.  St. Bonaventure wrote "Saw a seraph with six fiery and shining wings descend from the height of heaven.  And when in swift flight the seraph had reached a spot in the air near the man of God, there appeared between the wings the figure of a man crucified, with his hands and feet extended in the form of a cross and fastened to a cross.  Two of the wings were lifted above his head, two were extended for flight and two covered his whole body"�.  St. Bonaventure continued "mysterious and intimate conversation"� occurred between the seraph and Francis, leaving Francis "aglow with seraphic love in his soul".

After the encounter with the Seraphim ended, St. Francis discovered that the seraph had miraculously given him wounds reminiscent of those that Jesus Christ suffered while sacrificing himself for the world's sins in the cross.  St. Francis emerged from the encounter with the stigmata (five wounds) that Jesus suffered during His Crucifixion on His Body.  What was important is St. Francis was the first Saint to receive the stigmata by a Seraphim Angel.

Second
The second account involved Isaiah the prophet.  Isaiah described  his experience with a Seraphim in
Isaiah, Chapter 6 verse 1- 7 in the Bible
Call of Isaiah
One of the Seraphim flew to Isaiah, holding a live coal, an ember which he had taken with tongs from the altar.  The Seraphim touch Isaiah's with the live coal, the ember and The Seraphim said:

"See", "now that this has touched your lips,    your wickedness is removed, your sin purge".

The Seraphim cleansed Isaiah from sin.

Third
The apparitions of God The Father of All Mankind on July 1, 1932.
God appeared to Mother Eugenia .  God came with the heavenly court a precession of the elect, the Seraphim.  Mother Eugenia written words

"Finally I began to hear singing.  Angels came to announce this glad arrival!  Their songs were so beautiful that I decided to note them down as soon as possible. This harmony ceased and then came a procession of the elect ... seraphim with Our God, our Creator and our Father".