My dear brother Mushi,
AsSalaamu 'alaykum warahmatullaah,
JazaakumAllaah khair for reminding us of our brothers and sisters who are suffering and dying in Palestine. No doubt, we should not forget about their dreadful situation and continuously make du'aa for them, as every reminder is a reminder too little.
Dear brother, i would like to mention however something that is first and foremost a reminder to myself and then to your goodself.
We know the Religion of Islaam is complete and there is no scope for further additions as the Messenger of Allaah - salallaahu 'alayhi wa sallam explained. He said that: " ... every newly invented matter is misguidance, and every misguidance is going astray ..." Thus we cannot add nor subtract anything from Allaah's beautiful Deen as this will conflict with His Laws and displease Him, while our aim as Muslims should be to please Him alone.
Hence all the rules and regulations for worship have been laid down and stamped, sealed and delivered to us by our beloved Prophet - salallaahu 'alayhi wa sallam. Thus it is only in our most best interest that we avoid falling into error by introducing or following newly invented matters. One such matter was the fast of Monday that you mentioned specifically so the du'aa of that day can be dedicated to the Muslims of Palestine.
There is nothing wrong at all for fasting, particularly on Monday. In fact, it is highly rewarding (as it was the Sunnah of the Prophet - salallaahu 'alayhi wa sallam), but for every righteous action to be accepted by Allaah it must fulfill two conditions: the intention of the act must be for the sake of Allaah alone; and it must be in accordance with the Qur'aan and the Sunnah.
We know that the Messenger of Allaah - salallaahu 'alayhi wa sallam - always advised and encouraged the Believers to make du'aa particularly during times of need, but he did not advise the Muslims to fast during such times, for such reasons. The Muslims of Makkah, 1400 years ago went through much hardship and persecution at the hands of the Quraiysh, yet those who migrated to Abbysinia were not advised to fast in reminder of their brothers and sisters being oppressed in Makkah. Or perhaps later, when the verses of fasting were revealed in Madeenah, there were times when pockets of Muslims outseide the city were still suffering and being tortured by their enemies, yet the Prophet - salallaahu 'alayhi wa sallam - did not order the Muslims to fast in their remembrence. For sure supplications to Allaah were made for their security and success, but fast was not observed as a ritual.
We should be careful my dear brother, so as not to make certain events a reason for our worship (if it is not according to the Sunnah), but rather make our worship for the sake of Allaah alone and with the intention of practicing the Sunnah, and then make du'aa for our oppressed brothers and sisters in Palestine. As the Prophet - salallaahu 'alayhi wa sallam - said in the first hadeeth of Bukhaaree, "Verily every action is judged by intentions ..."
Even if ones intention is for the sake of Allaah, by asking everyone to fast on any specific day so that du'aa can be made for a specific reason is going astray as it is a newly invented matter into the Deen and is not from the Sunnah.
... and Allaah - subhaan wa ta'aala - knows best.
And we ask Him - the Almighty & Most Merciful - that He rectifies the condition of our brothers and sisters in Palestine and gives then patience in such testing times. And we ask Him alone that He grants them victory over the enemies of His Deen and the enemies of the Believers and makes them from amongst the successful both in this world and the Hereafter.
kind regards,
&peace