Here Allah (swt) literally says “I do not swear” as He wants to make it clear that the disbelievers have no right to deny the Day of Resurrection. So, the meaning of ”I do not swear” is that this matter is so clear that He (swt) does not need to swear in order to prove it, but if He (swt) were to swear, He would swear by the Day of Resurrection, as He only swears by something great.