When do the first and second “hours” on Friday begin?

It was narrated that Abu Hurayrah (may Allaah be pleased with
him) said: The Prophet (peace and blessings of Allaah be upon him)
said: “Whoever comes (to the mosque) in the first hour, it is as if he
sacrificed a camel. Whoever comes at the second hour, it is as if he
sacrificed a cow. Whoever comes at the third hour, it is as if he sacrificed
a horned ram. Whoever comes at the fourth hour, it is as if he sacrificed a
chicken. Whoever comes at the fifth hour, it is as if he sacrificed an egg.
Then when the imam comes out, the angels come in to listen to the reminder
(khutbah).” 

Narrated by al-Bukhaari, 841; Muslim, 850 

The scholars differed as to the definition of these hours.
There are three opinions. 

1 – That they start when dawn breaks

2 – That they start when the sun rises. This is the view of
al-Shaafa’i, Ahmad, and others.

3 – That these ‘hours’ all fall within one period of time,
which is after the sun has passed its zenith. This is the view of Maalik,
and was favoured by some of the Shaafa’is. 

The third view is weak, and was refuted by many. 

Al-Nawawi (may Allaah have mercy on him) said: 

It is well known that the Prophet (peace and blessings
of Allaah be upon him) used to go out to offer the Jumu’ah prayer straight
after the sun passed its zenith, as do all imams in all regions. That is
after the end of the sixth hour, which indicates that the one who comes
after the sun has passed its zenith has no share of guidance and virtue, and
no reward will be recorded for him at all, because he has come after the
scrolls (of the recording angels) have been rolled up. The mention of these
hours is intended only to encourage people to come early and attain the
reward of being in the first row and waiting for the prayer, and keeping
busy with naafil prayers, dhikr and the like. None of that can be achieved
by going to the mosque after the sun has passed its zenith, and there is no
virtue in doing so, because the call to prayer is given at that time and it
is haraam to delay responding to it. End quote. 

Al-Majmoo’, 4/414 

Ibn Qudaamah (may Allaah have mercy on him) said: 

With regard to the view
of Maalik that goes against these reports: it is mustahabb to perform
Jumu’ah prayer just after the sun has passed its zenith. The Prophet
(peace and blessings of Allaah be upon him) used to pray Jumu’ah early, and
when the imam comes out the scrolls (of the recording angels) are rolled up,
and no reward is written down for the one who comes to Jumu’ah prayer after
that, so what virtue can there be in such a person? End quote. 

Al-Mughni, 2/73 

The correct view is the
second one, which says that these hours begin when the sun rises, and the
time between sunrise and the second adhaan should be divided into five
parts, of which each one is what is meant by the word saa’ah (lit. “hour”)
in the hadeeth. 

Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have
mercy on him) was asked: 

When does the first hour of Friday begin? 

He replied:

 The hours which were mentioned by the Prophet (peace
and blessings of Allaah be upon him) are five. He said: “Whoever comes (to
the mosque) in the first hour, it is as if he sacrificed a camel. Whoever
comes at the second hour, it is as if he sacrificed a cow. Whoever comes at
the third hour, it is as if he sacrificed a horned ram. Whoever comes at the
fourth hour, it is as if he sacrificed a chicken. Whoever comes at the fifth
hour, it is as if he sacrificed an egg. Then when the imam comes out, the
angels come in to listen to the reminder (khutbah).” So the time between
sunrise and the imam’s arrival is divided into five parts, each of which may
be equivalent to what we know as an hour (sixty minutes), or it may be more
or less, because time changes. So there are five hours between sunrise and
the imam’s arrival for the prayer, starting at sunrise. It was also said
that it begins at the break of dawn, but the first view is more correct,
because the time before sunrise is the time for Fajr prayer. End quote. 

Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen,
16; question no. 1260 

For more details on this issue, please see Zaad al-Ma’aad
by Ibn al-Qayyim, 1/399-407. 

And Allaah knows best.