Repentance and expiation are required from one who commits zina during the day in Ramadan
Question

                Three or four years ago I committed the sin of zina during the day in Ramadan (I ask Allah for forgiveness). Please note that I was not praying or fasting; I only fasted in front of people and I only prayed when I had no option but to pray in the presence of people who are older than me. I am single and, praise be to Allah, I have repented to Allah, may He be exalted, the Most Merciful, the Forgiving, the Oft-Forgiving, may He be glorified.

Do I have to offer expiation for the sin of zina during the day in Ramadan?
Praise be to Allah.
    We praise Allah, may He be 
    exalted, for having guided you to repent from this grave sin. Whoever 
    violates the sanctity of the sacred month by committing the sin of zina, 
    repentance is a blessing from Allah, may He be exalted, that calls for 
    gratitude and appreciation. The greatest act of gratitude for it is to 
    adhere to it and persist in it, to continually ask for forgiveness and to 
    regret what has happened in the past, as well as asking Allah, may He be 
    exalted, in all places and at all times when supplication is answered, for 
    pardon and forgiveness, in the hope that Allah will accept his repentance 
    and forgive what has happened in the past. Allah, may He be exalted, says 
    (interpretation of the meaning):

    “O you who believe! Turn to 
    Allah with sincere repentance! It may be that your Lord will remit from you 
    your sins, and admit you into Gardens under which rivers flow (Paradise) the 
    Day that Allah will not disgrace the Prophet (Muhammad SAW) and those who 
    believe with him, their Light will run forward before them and with (their 
    Records Books of deeds) in their right hands they will say: "Our Lord! Keep 
    perfect our Light for us (and do not put it off till we cross over the Sirat 
    (a slippery bridge over Hell) safely) and grant us forgiveness. Verily, You 
    are Able to do all things”

    [at-Tahreem 66:8]. 

    With regard to expiation, it is 
    obligatory for you, according to the consensus of the fuqaha’ of the four 
    madhhabs. They have stated that expiation for intercourse during the day in 
    Ramadan is obligatory in cases such as yours, whether that intercourse took 
    place within the framework of a permissible relationship or a forbidden 
    relationship. If it took place within the framework of a forbidden 
    relationship, then the sin of immoral conduct and transgressing the limit 
    set by Allah is added to the sin of violating the sanctity of the fast and 
    the sacred month. 

    Al-‘Imraani (may Allah have 
    mercy on him) said: 

    If he commits zina with a woman 
    during the day in Ramadan… he has broken the fast [and he stated that 
    expiation is obligatory].

    End quote from al-Bayaan fi 
    Madhhab al-Imam ash-Shaafa‘i (3/525). 

    The scholars of the Standing 
    Committee for Issuing Fatwas said: 

    The Prophet (blessings and peace 
    of Allah be upon him) stated the ruling that expiation was obligatory for a 
    Bedouin because he had intercourse with his wife deliberately during the day 
    in Ramadan when he was fasting. That was an explanation on his part 
    (blessings and peace of Allah be upon him) of the reason for the ruling. The 
    fuqaha’ are unanimously agreed that the fact that the story refers to the 
    Bedouin having had intercourse with his wife implies that expiation is also 
    required in the case of intercourse with a slave woman and in the case of 
    zina. They are also unanimously agreed that the fact that the one who had 
    intercourse came and expressed regret does not have any impact on the 
    obligation of offering expiation.

    End quote from Fataawa al-Lajnah 
    ad-Daa’imah (10/300) 

    ‘Abd al-‘Azeez ibn Baaz, ‘Abd 
    ar-Razzaaq ‘Afeefi, ‘Abdullah ibn Ghadyaan 

    Finally, we advise you to fear 
    Allah, may He be exalted, in private and in public, and to repent from 
    having not prayed and fasted in the past, and to strive hard to make up what 
    you have missed of those actions at the earliest opportunity. 

    For information on expiation and 
    the rulings thereon, please see the following questions: 
    1672, 22938, 38023 and
    39734 

    And Allah knows best.