When should the Peruzbal be performed?
Initially the Peruzbal is to be performed at the end of the sixth year, prior to Rosh Hashanah of the seventh year. [It is a Mitzvah to publicize this matter as it is not well known to the public. It is proper to arrange the performance of the Peruzbal on Erev Rosh Hashanah immediately after Hataras Nedarim.] If one did not do the Peruzbal at the end of the sixth year then he is to perform it anytime throughout the seventh year, up until Rosh Hashanah which is the beginning of the eighth year. [Practically, the Chabad custom is to perform the Peruzbal both on Erev Rosh Hashanah of the sixth year and the seventh year. This certainly applies if one made loans within the 7th year of which their due date arrived, and they have not yet been paid.]
What does one do if he forgot to make a Peruzbal at the end of the 6th year, may he still collect the loans?
Yes. See Halacha 4 in Q&A!
 Admur Halvah 36
 Admur ibid; Rosh Gittin 4:18; Rabbeinu Yerucham Nesiv 7; Haitur Peruzbal; Reb Shimon Duran, brought in Radbaz 2238; Tur 67; Kaftor Vaferach 50; Tumim 67:54; Misgeres Hashulchan 180:9; Mahraiy Asad 2:79; Peas Hashulchan 97 that so ruled Gr”a; See Nitei Gavriel Peruzbal 15:2
The reason: As there are opinions [Rosh ibid] which hold that although the seventh year only annuls the loan at the end of the year, as the verse states “At the end of seven years perform an annulment”, nevertheless once the Shemita year has begun one is no longer allowed to demand payment of the debts. This is based on the verse “Do not demand payment from your friend as it is a Shemita for Hashem”. From here it is learned that as soon as the year is called Shemita it is forbidden to demand payment. Nevertheless, even according to this opinion, there is a difference between the start of the seventh and the end of the seventh as if the borrower gives back the money within the Shemita year then the lender may accept it and is not required to say, “I forgive the loan.” However, after the Shemita year has passed if the borrower comes to repay the loan the lender is required to tell him “I forgive the loan”. This is learned from the verse “Vizeh Dvar Hashemita….” which implies that one must verbalize with his mouth the nullification of the loan, however this itself must only be done at the end of the seventh year. [Admur ibid]
Other opinions: Some Poskim rule one is to perform the Peruzbal only towards the end of the 7th year, as if he does prior to the 7th year all the loans made after the Peruzbal will not be included in it and it will be forbidden for him to collect money. [Radbaz 2238 in negation of opinion of Reb Shimon Duran]
 Likkutei Sichos 7:355; 24:316; Gloss of the Rebbe printed in Sefer Haminhagim p. 197 [English]; Shulchan Menachem 7:44; See Nitei Gavriel Peruzbal p. 15 that he was told by Rav S”Z Aurbach that after receiving the directive of the Rebbe, Rav A”C Naah publicized in 1952 to write the Peruzbal on Erev R”H of the end of the 6th year as rules Admur ibid; The Minchas Yitzchak told him that this was the custom of Chabad of Manchester in 1959 and 1966 based on the directive of the Rebbe. The Pnei Menachem of Gur told him that the Rebbe did a great thing by publicizing this matter as when we annul the debts below in this world so too Hashem annuls the debts of the Jewish people in Heaven. See also Sichas Kodesh 1979 3:750.
 Although it may be performed on any day prior to the start of the 7th year, nevertheless since the Peruzbal only works for previous loans therefore it is to be done on the last day of the sixth year in order to cover all the loans of that year. If one performed the Peruzbal prior to Erev Rosh Hashanah, then all loans that take place after the Peruzbal was done are not included within the allowance of collecting the debt accomplished by the Peruzbal. [Likkutei Sichos 7:355]
 Likkutei Sichos 7:356; 24:316; The Rebbe’s custom was to perform the Peruzbal after Hataras Nedarim.
The reason: Although it is more logical to perform the Peruzbal as close to sunset on Erev R”H as is possible in order to cover any loans that were made that day, nevertheless since people may forget to do so and by Hataras Nedarim there is already a tribunal of three Kosher Jews convened, therefore one should do it at this time. [ibid]
 Luach Kolel Chabad; Custom of the Rebbe in 1970 and 1988 which were Erev R”H of the end of the seventh year; Hayom Yom [printing of 1989] “In the end of 1988 the Rebbe requested that people recite the Peruzbal at the end of the seventh year”; See Otzer Minhagei Chabad p. 41; Shevach Hamoadim footnote 11
The reason: The Rebbe stated the reason he made a Peruzbal also by Erev Rosh Hashanah of the end of the seventh year is in order to go according to all opinions. [Hayom Yom ibid] There are opinions that rule that those loans which are given in the seventh year itself also require a Peruzbal in the end of the seventh year even if one already made a Peruzbal in the end of the sixth. [Bach 67; See Nitei Gavriel Peruzbal chapter 15:4] However from the wording in Admur ibid that “if he did not do so by the end of the sixth year then he should do so in the 7th” implies that if one did do the Peruzbal in the 6th year there is no need to do so in the 7th. [See Nitei Gavriel ibid] Perhaps however no inference can be made from here as Admur is referring to even one that did not make any loans in the 7th year that nevertheless Bedieved he can still make a Peruzbal in the seventh year for the loans of the previous years. If, however, in truth one made a loan in the seventh year then he would be required to make another Peruzbal by the end of the 7th year even if he already made one in the end of the 6th year. So also concludes the Rebbe in Likkutei Sichos 12:256, although he leaves the stringent opinion of the Rosh and Tur with a Tzaruch Iyun as how they would rule regarding loans of the 7th year.
 Likkutei Sichos 12:256 in answer to the question of what one is to do if he made loans in the 7th year. See previous footnote!