On Shiva Asar B’Tammuz we don’t fast until the morning. Does that mean that the night before is not part of the Taanis and the halachos of the three weeks don’t start until the morning, or does the three weeks start as soon as it becomes night on 17 Tammuz but the actual not eating only begins in the morning?
Rav Moshe Feinstein in Igros Moshe (OC 1:168) says that it is a Machlokes Rishonim. Rav Moshe brings proof from the gemara that the taanis only starts in the morning but he says because of his proof we cannot pasken between the opinions of the rishonim. Nevertheless says Rav Moshe, these issurim of shaving, weddings, and the halachos of the three weeks are not Din but rather Minhagim of Aveilus during a bad period for Am Yisroel, therefore we can be more lenient. Additionally he suggests that maybe even the Rishonim who hold the Taanis starts at night would agree that these things only become forbidden in the morning when the actual fasting begins.
Therefore Rav Moshe says that in a Makom Tzorech it is permissible to treat the night of Shiva Asar B’Tammuz as if the three weeks have not started.