1. "The laws of Sabbath are suspended in the face of a danger to life (piquah nefesh) Therefore, we may perform according to the directives of a physician everything that is necessary for the benefit of a sick person whose life is in danger." (Maimonides MT, Shabbat 2:1)
This principle is learned from the verse in Lev 18:5. HaShem said: "You shall keep My statutes and my rules; which when a person does them, he lives by them... ". Our rabbis explained: when by performing these laws "he lives" he must observe them. However, when by keeping these Mitsvot he or she would die, then the Mitsvot are suspended (except cases of yeherag ve-al ya'abor.Example: if someone tells X: kill Y or else I kill you, X cannot kill Y to save his own life)
2. Even when there is a doubt or a slight possibility to save someone's life Shabbat must be suspended.
"When there is a doubt whether or not the Sabbath must be violated on a person's behalf, one should violate the Sabbath laws on his or her behalf, for the Sabbath laws are suspended even when there is merely a possibility of danger to a person's life (safeq piquah nefesh). The same principles apply when one physician says that the patient is not in danger and another physician says that he is in danger, the Sabbath should be violated on that person's behalf." (Idem)
Illustrations: 1. If there is a medication or an experimental drug that has been proven to help some patients, one should suspend the laws of Shabbat if necessary to get that of medication, even when one is not positive it will save the patient's life. 2. If a building collapsed, and there is uncertainty if a person lies under the debris or if we are uncertain if that person is still alive, the laws of Shabbat should be suspended to try to save a life.
Candle lighting in NYC: 6:14
Shabbat ends in NYC: 7:12