Question: What is the importance of the time of sahar (last portion of the night)? How is it calculated?
ANSWER
The time of sahar is the final one-sixth of the night. That is, it is the final one-sixth of the duration from sunset to the time of imsak (the time when prohibition on eating begins). Let us say for example that the sun sets at 5:30 p.m. and the time of imsak occurs at 5:30 a.m., the night is 12 hours long. The one-sixth of this equals 2 hours. When it is subtracted from 5:30 a.m., it is 3:30 a.m. The time of sahar is from 3:30 a.m. to 5:30 a.m. This duration increases or decreases in summer and winter.

It is better to perform the namazes of tahajjud (voluntary late night namaz) and witr at the time of sahar. It is declared in hadith-i sharifs:
(Whoever fears that he will not be able to wake up in the last portion of the night, let him perform the witr in the early portion of the night. Then let him go to sleep. Whoever thinks that he will be able to wake up in the last portion of the night, let him perform the witr then because Angels of Compassion are present when he wakes up for it in the last portion of the night.) [Muslim]

(The du'a [supplication] made in the last portion of the night and the du'a made after namazes are accepted.) [Tirmidhi]

(Allahu ta'ala declares in the last portion of the night: "Isn't there anyone to make repentance, so that I shall forgive him? Isn't there anyone to ask, so that I shall give what he wants and accept his supplication?") [Muslim]

The time of sahar is a time when supplications and repentance are accepted. When one gets up for sahur (pre-dawn meal) in Ramadan, one will have gotten up at the time of sahar. One should deem it a great opportunity to make supplication at this time. In praising good people [in the Qur'an al-karim], Allahu ta'ala says, "They ask for forgiveness in the last portion of the night" (Adh-Dhariyat 18).

Ya'qub alaihis-salam said to his sons, "Soon [at the time of sahar], I will ask forgiveness for you from my Lord" (Yusuf 98).

In the 17th verse of Al-i Imran Sura, those who show patience, those who are truthful [in their speeches, deeds, and intentions], those who perform namaz, those who give zakat, and those who ask forgiveness in the last portion of the night are praised. The reason why those who ask forgiveness are mentioned after all others is that humans should consider their every act of worship as defective and should always implore for forgiveness.

Opportunity is a windfall. We should not waste our lifetimes on useless pursuits. Instead, we should spend them on things Allahu ta'ala is pleased with. We should perform five daily namazes with tadil-i arkan and in congregation, offer the namaz of tahajjud, and should not let the last portion of the night pass without imploring for forgiveness. We should not be overcast with the slumber of oblivion, but meditate death and afterlife. Turning away from prohibited worldly affairs, we should direct ourselves towards otherworldly deeds. We should engage in worldly earnings as much as indispensably necessary, and at other times we should fully occupy ourselves with making our afterlife better. (Maktubat-i Ma'thumiyya)

Question: Is it sinful to fast without getting up for sahur (pre-dawn meal)?
ANSWER
It is not sinful, but it is much reward-deserving to get up for pre-dawn meal. It is good to get up for it even if one gets up just to drink a sip of water. It is declared in hadith-i sharifs:
(Sahur meal is blessed. The whole of sahur is blessing. Get up for sahur even if you drink just a sip of water. Allahu ta'ala and His angels shower salat and salam upon those who get up for sahur.) [Imam-i Ahmad] (That is, Allahu ta'ala forgives those who get up for sahur and angels pray for them.)

(Get up for sahur; there is blessing in it.) [Bukhari]

(Aid yourselves in fasting by eating a meal at sahur.) [Bayhaqi]

(Getting up for sahur meal is a blessing that Allah has bestowed upon you. Do not miss it.) [Nasai]

(Providing that what they eat is halal [lawful], three people will not be called to account: a person who fasts, a person who eats sahur meal, and a person who keeps guard in the way of Allah.) [Nasai]

(Certainly sahur meal is blessed.) [Ibn Hibban]

(Have sahur meal even if it is just a morsel because there is blessing in it.) [Daylami]

(How excellent are dates as a Believer's sahur.) [Abu Dawud]

(Allahu ta'ala showers mercy upon those who get up for sahur.) [Tabarani]