Taqabbal Allāhu minnā wa minkum ṣālihal-aʿmāl
Islam21c can confirm that Eid al-Fiṭr 1440 will be on Tuesday – corresponding to 4th June 2019 – based on an official declaration of the Supreme Court of the Kingdom of Saudi Arabia.
As the majority of Muslims in the UK follow Saudi Arabia in starting and ending Ramaḍān, tomorrow will be the day of Eid.
At the time of our announcement Islam21c confirmed negative sightings from the Sudayr region of Saudi Arabia, however there have been several reports of positive sightings from various other stations in Saudi Arabia since. This resulted in some confusion and conflicting reports, but the Supreme Court of the Kingdom of Saudi Arabia has now officially announced Eid for tomorrow. 
May Allāh (subḥānahu wa taʿālā) accept our ṣiyām, our qiyām, our duʿā’, our charity, and all our ʿibādah, Āmīn.
ʿAbdullāh b. ʿUmar (raḍiy Allāhu ʿanhu) reported Allāh’s Messenger (sall Allāhu ʿalayhi wa sallam) as saying:
The month (of Ramaḍān) may consist of twenty nine days; so when you see the new moon observe fast and when you see (the new moon again at the commencement of the month of Shawwāl) then break it, and if the sky is cloudy for you, then calculate it (and complete thirty days). 
Based on this narration, since, the crescent of 1st of Shawwāl 1440 has been sighted tonight, we will end the month of Ramaḍān 1440 on Monday at Maghrib and celebrate Eid al-Fiṭr on Tuesday, in shā’ Allāh.
Don’t forget to pay your Zakāt al-Fiṭr! This is a small donation that is to be given before the Eid ṣalāh. If you have not paid Zakāt al-Fiṭr for any members of your family, please do so here. If you have any questions on Zakāt al-Fiṭr, please click here to read our Zakāt al-Fiṭr Q&A by Sh (Dr) Haitham al-Haddad.
The Islam21c team would like to take this blessed occasion to wish you all Eid Mubārak! We pray that Allāh (subḥānahu wa taʿālā) bestows the Muslims across the globe a safe, secure, and blessed Eid. May Allāh (subḥānahu wa taʿālā) have mercy on those who are suffering, grant them patience, victory, and bring them justice in the dunyā and ākhirah, Āmīn.
Notes: https://mobile.twitter.com/spagov/status/1135593699964940291  Saḥīḥ Muslim