The task of this publication was to present well known law term vacatio legis in two meanings: narrow (as an adaptive period) and wide (as legal institution). With the analysis of phrases related to subjective term, such as: official announcement, entry into force, binding force and observance of law, there has been shown a few problems connected to temporal range of being in force by the law, also on a ground of changes brought by amendment of principles of legislative technique, that are in force since 1st March 2016. Very interesting opinions arise from Polish Constitutional Courts and scientists statement with reference to the rule of proper period of rest. They emphasize and justify a claim that vacatio legis as legal institution, without any doubts, serves the protection of the rule of citizens’ trust to state and law, and thus it guarantees constitutional rule of democratic state of law.