When to Do Ghusl for Jumu`ah
Question

1- Is it valid for one to have a bath after Maghrib on Thursday, is it sufficient enough for him for the Friday or must he have a bath again in the morning of Friday?
2- The same question in regard to reading surah Al-Kahf on Thursday evening, or is there any specific timing which the surah should be read on Jumu`ah ?
Praise be to Allah.Time for doing Ghusl on Friday 
The scholars differed as to when the time for doing Ghusl on Friday begins. The majority (including the Shafi`is, Hanbalis and Dhahiris) were of the view that it begins from dawn on Friday. This was narrated from Ibn `Umar (may Allah be pleased with him). 
Al-Khatib Ash-Sharbini (may Allah have mercy on him) said concerning the time for doing Ghusl on Friday :
“The time for that is the true dawn, because the reports connected it to the day, such as the words of the Prophet (blessings and peace of Allah be upon him): “Whoever does Ghusl on Friday then sets out in the first hour…” So it is not valid if done before then. 
And it was said: that the time for it begins halfway through the night as is the case on Eid. 
Doing it close to the time when one goes for Jumu`ah prayer is better, because it is more likely to achieve the purpose of eliminating bad smells. If there is a conflict between doing Ghusl and going early to the prayer, then paying attention to Ghusl takes priority, as Az-Zarkashi said: because there is a difference of opinion as to whether it is obligatory.” (Mughni Al-Muhtaj, 1/558) 
Al-Bahuti Al-Hanbali (may Allah have mercy on him) said: 
“The beginning of its time is the break of dawn; doing Ghusl before then is not valid…  but it is better to do Ghusl just before going to Jumu`ah , because that is closer to achieving the purpose.” (Kashshaf Al-Qina’, 1/150) 
Ibn Hazm (may Allah have mercy on him) said: 
“The first of the times for the Ghusl mentioned is just after dawn breaks on Friday. … we have narrated from Nafi’ from Ibn ‘Umar that he used to do Ghusl after dawn broke on Friday, and that is acceptable as the Ghusl for Jumu`ah. It was narrated from Mujahid, Al-Hasan and Ibrahim An-Nakha’i that they said: If a man does Ghusl after dawn breaks, that is acceptable.” (Al-Muhalla, 1/266) 
According to Imam Malik (may Allah have mercy on him), the time for Ghusl on Friday begins just before going to the prayer, and it is not acceptable except at the time of going; if a person does Ghusl after dawn and does not go to the mosque straightaway, that is not acceptable and it is recommended for him to repeat it. 
An-Nawawi (may Allah have mercy on him) said:
“If he does Ghusl for Friday before dawn, that is not acceptable according to the sound opinion of our madhhab, and this is also the view of the majority of scholars, but Al-Awza’i said that it is acceptable. 
If he does Ghusl for Jumu`ah after dawn has broken, it is acceptable in our view and in the view of the majority. This was narrated by Ibn Al-Mundhir from Al-Hasan, Mujahid, An-Nakha’i, At-Thawri, Ahmad, Is-haq and Abu Thawr (may Allah have mercy on them). 
Malik (may Allah have mercy on him) said: It is not valid except when going to Jumu`ah. 
All of them say that it is not acceptable before dawn, apart from Al-Awza’i who said: it is acceptable for him to do Ghusl before dawn for both janabah and Jumu`ah.” (Al-Majmu`, 4/408; see also Hashiyat Al-‘Adawi, 1/379; At-Taj wa’l-Iklil, 2/543) 
The first view (that the time for it begins at the break of dawn) was favoured by Shaykh Ibn Baz (may Allah have mercy on him). 
Shaykh Ibn `Uthaymin (may Allah have mercy on him) said: 
“Ghusl for Jumu`ah begins at the break of dawn, but it is better not to do Ghusl until after the sun has risen, because the day has certainly begun after sunrise, and because the time before sunrise is the time for Fajr prayer, and the time for Fajr prayer has not ended yet, so it is better not to do Ghusl until after the sun has risen. Then it is better not to do Ghusl until the time for going to Jumu`ah, so that he will be going to Jumu`ah immediately after purifying himself.” (Majmu` Fatawa Ibn `Uthaymin, 16/142) 
When to read Surat Al-Kahf on Friday 
With regard to the time when it is recommended to recite Surah Al-Kahf on Friday , it is from sunset on Thursday until sunset on Friday, because the Hadiths encourage reading it on Friday and the night of Friday (i.e., the night before Friday). 
And Allah knows best.