We know that killing is happening on a wide scale today in the land of Palestine, and Muslims in the rest of the world are torn between sorrow and being unable to help. My question is: is it permissible for the Muslims not to celebrate Eid because of what is happening of tragedies in Palestine, especially if the Muslim feels grief and sorrow because of that?

Praise
be to Allah

The Eids of the Muslims have not been prescribed only for the
purpose of joy, fun and visiting one another; rather they are among the
symbols and acts of worship of the religion, so the Sunnah on these
occasions is for the Muslims to celebrate them openly and proclaim them. 

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him)
said:

The Eids are among the most specific distinguishing features
of different religions, and among the most obvious of their rituals.

End quote from Iqtida’ as-Siraat al-Mustaqeem (1/528)

Hence you will find that every religion has its own festivals
that its followers care about and celebrate openly, because they are an
important part of their religion.

Al-Haafiz Ibn Hajar (may Allah have mercy on him) said:

Expressing joy on the occasion of the Eids is one of the
symbols of the religion.

End quote from Fath al-Baari (2/443)

So expressing joy on Eid is one of the acts of worship by
which the Muslim draws closer to Allah.

Ahmad (24334) narrated that ‘Aa’ishah (may Allah be pleased
with her) said: The Messenger of Allah (blessings and peace of Allah be upon
him) said on that day – i.e., the day on which the Ethiopians played in the
mosque: “Let the Jews know that in our religion there is lenience. Verily I
have been sent with an easygoing religion.”

Classed as saheeh by al-Albaani in Saheeh al-Jaami‘
(3219).

Moreover, there is no contradiction between expressing joy on
Eid and feeling sorrow for what has befallen the Muslims and grieving for
their situation. The Muslim expresses his joy on Eid in order to show his
religious commitment openly and to show his support for his religion and at
the same time he shares the sorrows of the Muslims.

So the Muslim should combine both matters: he should practice
the rituals and acts of worship of Islam openly, such as the Eid prayer, and
he should express some measure of joy and happiness. And at the same time he
should feel sorrow for what has befallen his brethren, and he should feel
their pain. 

Undoubtedly the more the Muslim feels the pain and anguish of
his Muslim brothers, the less he will indulge in permissible kinds of
entertainment and play, even if he allows himself expressions of joy on Eid
and gratitude for the blessings that Allah has bestowed upon him. 

See the answer to question no. 21284
for more information on what we must do with regard to the slaughter of
Muslims around the world.

See the answer to question no. 7837
for information on how to console our brethren who are stricken by
calamity. 

And Allah knows best.