On the feast of Stephen

"Good King Wenceslas last looked out,
On the feast of Stephen,
When the snow lay round about
Deep and crisp and even.
Brightly shone the moon that night
Though the frost was cruel,
When a poor man came in sight
Gathering winter fuel."

Wenceslaus Square, Prague

He was a real person, and very important in the history of Prague. The website Private Prague Guide says:

The first Czech saint and the patron saint of the Czech state, Wenceslas (Václav in Czech) served as duke of Bohemia from 921 until his death in 929 or 935. Though he died young, this martyr’s accomplishments were many. He built numerous churches in Bohemia and was deeply respected as a pious, moral, educated and intelligent man who promoted the Christian faith and took care of the poor, the sick, the widowed and the orphaned by doing charitable deeds. He even founded the rotunda of Saint Vitus at Prague Castle.

The whole carol:

Good King Wenceslas looked out
On the feast of Stephen
When the snow lay round about
Deep and crisp and even
Brightly shone the moon that night
Though the frost was cruel
When a poor man came in sight
Gath'ring winter fuel

"Hither, page, and stand by me
If thou know'st it, telling
Yonder peasant, who is he?
Where and what his dwelling?"
"Sire, he lives a good league hence
Underneath the mountain
Right against the forest fence
By Saint Agnes' fountain."

"Bring me flesh and bring me wine
Bring me pine logs hither
Thou and I will see him dine
When we bear him thither."
Page and monarch forth they went
Forth they went together
Through the rude wind's wild lament
And the bitter weather

"Sire, the night is darker now
And the wind blows stronger
Fails my heart, I know not how,
I can go no longer."
"Mark my footsteps, my good page
Tread thou in them boldly
Thou shalt find the winter's rage
Freeze thy blood less coldly."

In his master's steps he trod
Where the snow lay dinted
Heat was in the very sod
Which the Saint had printed
Therefore, Christian men, be sure
Wealth or rank possessing
Ye who now will bless the poor
Shall yourselves find blessing 

I especially remark the last two lines of the final stanza.