In the wake of the events in the Kerch Strait, President Poroshenko met with his National Security and Defence Committee, who decided that, because of this and other information they had in their possession suggesting a possibly imminent Russian land invasion, Martial Law should be declared.
Originally, Poroshenko wanted the period of Martial Law to be two months and for it to cover the entire country. His proposal was given the thumbs up by his allies on the NSDC, but parliament had other ideas. Once made public, some people smelled a rat, as the two-month period the President was asking for would, maybe conveniently or maybe coincidentally, just reach beyond the threshold for any last remaining candidates to nominate themselves for the upcoming presidential elections. If, for example, a super popular (and popular and populist are not the same thing) candidate had not yet declared, imposing martial law for two months might kneecap their chances of getting on the ballot.
Some political horse-trading then brought the term down from two months to one, and also curtailed the geographical areas in which Martial Law would be applied. Basically, the ten regions of Ukraine that border Russia and the southern seas, as well as the Russian occupied enclave of Transnistria and Belarus (both places where Russia has a lot of troops) are covered by what is essentially a heightened state of readiness. Reports from those areas say not much has changed and citizens go about their daily business without hindrance, but there is a more visible military presence on the streets. Just in case.