Praise be to Allah, the Lord of all worlds, and may peace and blessings be upon our Prophet Muhammad and his family and companions.
The Day of Arafah is the greatest day of the year. This is evident for those who are performing Hajj. The Prophet (ﷺ) considered it for them as the Hajj. He (ﷺ) said, Hajj is Arafah. This means that it is the key and central-most pillar of Hajj.
‘A’isha (radiy Allāhu ‘anha) narrated that the Messenger (ﷺ) of Allah said,
“There is no day on which Allah frees more people from the Hellfire than the Day of Arafah.
“He comes close to those standing on the plains of Arafah and then boasts about them to the angels, saying:
‘What do these people seek?'” 
Furthermore, Abdullah ibn Amr ibn al-Ās (may Allah be pleased with them all) reported that the Prophet (ﷺ) said,
“The best supplication is the supplication on the Day of Arafah, and the best thing that I and the prophets before me have said is:
‘There is no deity worthy of worship except Allah, alone, without partner. To Him belongs the dominion, and to Him belongs all praise, and He is capable of all things.'” 
In addition, Abu Qatādah (radiy Allāhu ‘anhu) reported,
“The Messenger (ﷺ) was asked about the observance of fasting on the Day of Arafah.
“He (ﷺ) said,
‘It is an expiation for the sins of the preceding year and the current year.'” 
Dhikr and du’ā on Day of Arafah hold greater virtue than other days
Scholars differ regarding whether these virtues are exclusive to those performing Hajj or whether they are applicable to all Muslims. However, it is certain that supplications and remembrance of Allah on this day hold greater virtue than on other days.
Reports indicate that some early generations permitted congregating in mosques for supplication and remembrance of Allah on the Day of Arafah. Ibn Abbas (radiy Allāhu ‘anhu) practised this along with some other Muslims, and although Imam Ahmad permitted it, he himself did not engage in it. This is known as tarīf.
It is different from the controversial practice that some Muslims used to do, where they attended the masjid with the ihram/ Hajj clothes. Here, we are only talking about coming to the masjid and doing dhikr and du’ā.
Recommendation for brothers
It is recommended for brothers to take the Day of Arafah as a day off, observe the fast, and attend the mosque at least from Duhr prayer onwards.
They should engage in the remembrance of Allah and supplication until Maghrib prayer, considering it a form of I’tikāf (seclusion for worship).
This is one of the most virtuous acts — waiting in the mosque for the subsequent prayer after completing the current one.
Abu Hurayrah (radiy Allāhu ‘anhu) reported that the Messenger (ﷺ) of Allah said,
“Shall I not tell you something by which Allah effaces the sins and elevates the ranks (in Jannah)?
“The Companions said, ‘Yes (please tell us), O Messenger of Allah.’
“He (ﷺ) said, ‘Performing the Wudū properly in spite of difficult circumstances, walking with more paces to the mosque, and waiting for the next Salat (prayer) after observing Salat; and that is al-Ribat, and that is al-Ribat.” 
Recommendation for sisters
As for women, they should also fast and engage in the remembrance of Allah and supplication at home.
They should offer Duhr prayer and remain in their designated place, engaging in the remembrance of Allah and supplication until Maghrib. The previous Prophetic statement is applicable for them as well.
It is advisable to prepare food one or two days prior to the Day of Arafah, so our sisters will not be busy on the Day of Arafah (in preparing food).
Make the most of this gift
Brothers and sisters, make the most of the gift Allah has given you. It will not come again. The day that goes, will never come again.
The truly intelligent person is the one who is constantly preparing for his real life after death.
Finally, it is essential for all brothers and sisters to supplicate for the entire Muslim Ummah. May Allah accept our supplications and grant us repeated opportunities to perform the Hajj.