The conflict, if I have to choose a date when it began in earnest, began in the year 1921, on the day Palestinian Arabs attacked the immigrants' house in Jaffa. This attack, of course, had nothing to do with the territories or settlements. It was against the immigration of Jews to the Land of Israel.
And now, the official translation:
In my opinion, if one must choose a process by which the conflict started in actual fact, I would set the date at 1921 on the day on which the Palestinian Arabs attacked the immigration hostel in Jaffa. Many Jews were killed in this attack, including the well-known writer Y.H. Brenner. This attack was directed against Jewish immigration. My grandfather arrived in Jaffa, at that same hostel, the year before, as did many others. Clearly this attack was not about territory or settlements; it was against Jewish immigration to the Land of Israel...
Why didn't he choose April 1920? He may have chosen 1921 because he mentioned (here, at 1:05:20) that his grandfather had arrived there a year earlier. But during Pesach 1920, as reported:
Jews were killed and injured by Arabs, without any provocation (this: local Arab expectations had been raised to a pitch by the declaration of the Syrian Congress on 7 March of the independence of Greater Syria in the Kingdom of Syria, with Faisal as its king, that included the British-controlled territory within its claimed domain. On 7 and 8 March, demonstrations took place in all cities of Palestine, shops were closed and many Jews were attacked. Attackers carried slogans such as "Death to Jews" or "Palestine is our land and the Jews are our dogs!").
Ze'ev Jabotinsky organized a Jewish self-defense corps to protect the city's Jewish population. That alone would have been worthy enough to mention that violent murderous attack.
There were earlier attacks, mainly stone-throwing, at the early Moshavot. On March 29,1886, Arabs of Yehudiyeh attacked Petah Tikvah. In 1907, Arabs attacked and attempted to rape a Jewish girl out harvesting the wheat in the fields of Gedera (a previous attack happened on October 19, 1888). Moshe Shachvitz shot and killed the Arab and shortly thereafter, Arabs from his village, Katara, but where chased away. In 1890 at Yesud HaMaaleh, as a result of an altercation, a Sheikh from Safed who trespassed into a Jewish field was killed by a blow to the head. Zarnoga Arabs attempted to invade Rechovot on March 24,1892. At Rosh Pinna, Arabs threw rocks at Jews in the summer of 1883 in an attempt to take over a watering pool the pioneers had built whereas the Arabs of the Zangriya tribe had taken their flocks to the Jordan River. On December 6, 1896, there was an attack on the Jewish residents of Qastina.and the Jews of Hadera were attacked on September 12, 1901. In most of these instances, the Zionist Movement, another "guilty party" the Arabs accuse, hadn't been established. And the list goes on.
Here's a report from March 6, 1914 on Arabs shooting at Jewish homes at 10 PM at Kfar Uriyah:
Arab violence did not result from but from the Arab non-recognition of the Jewish national ethos, at all.