The filing deadline to submit your 2023 tax returns or an extension to file and pay tax owed is on Monday, April 15, 2024, for most taxpayers (if you live in Massachusetts or Maine, you have until April 17, 2024. This is because of the Patriots' Day holiday in those states).
If you have a refund though, the deadline is not applicable.
If you have to file only Form 8843, the deadline is the same as the one for filing form 1040NR.