Q. Is it OK to book plane tickets, hotels get passports and do the necessary arrangements, during the 3 weeks for a trip taken place right after Tisha b'Av ?
A. Shulchan Oruch (O.C. 551,2), rules that from Rosh Chodesh Av onwards we should minimize on commerce and dealing. Mishne Beruro (ibid. 11) quotes different opinions as to if it applies only to joyful and pleasant trades, or if all business dealings are included. However, he adds, that if needed and it is a unique saving opportunity, it is permitted. Prior to Rosh Chodesh, there in no proscribed Minhag.
Horav Shlomo Miller's Shlit"a opinion is likewise.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit"a