What is the ruling on having doubts about the Last Day?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.

If these doubts are just thoughts that you are striving to banish, then they do not blemish the soundness of your Eemaan (faith). Rather, feeling bothered by this is indicative of your belief. It is narrated on the authority of Abu Zumayl that he complained to Ibn ‘Abbaas about his doubts. The latter told him that they occur to everyone, and advised him – if he has them – to say, "He is (i.e. Allaah) the First and the Last, The Most High and The Most Near and He is, of all things, Knowing." [Abu Daawood]

Ibn ‘Abbaas narrated that a man came to the Prophet, sallallaahu ‘alayhi wa sallam, and asked him about the doubts he had. The Prophet, sallallaahu ‘alayhi wa sallam, told him that they were e of Satan’s plot. [Ahmad]

However, if these doubts are constant and you feel satisfied with them, without considering the gravity of airing them, then such a person becomes a disbeliever. This is because firm belief in the Last Day is an essential pillar of Eemaan (faith), which becomes unsound if even one pillar is deficient. Allaah The Almighty Says (what means): {But [true] righteousness is [in] one who believes in Allaah, the Last Day, the angels, the Book, and the Prophets.} [Quran 2:177]

‘Umar ibn Al-Khattaab narrated in  the Hadeeth of Jibreel that the Prophet, sallallaahu ‘alayhi wa sallam, said Eemaan includes believing in Allaah, His angels, His Books, His Messengers, the Last Day, and in fate, either good or bad.

Allaah Knows best.