The problem pointed by User 48152 can be resolved with the combined usage of textual criticism and context.
Starting with the first method (textual criticism), I now quote some samples of comments (by Bible scholars; from now on, the bold is added by me):
“The Textus Receptus has here a sentence not in the best MSS.: ‘I must by all means keep this feast that cometh in Jerusalem.’ This addition by D [= Bezae Codices] and other documents may have been due to a desire to give a reason for the language in Act 18:22 about ‘going up’ to Jerusalem. (Word Pictures in the New Testament, by A. T. Robertson)
“’I must by all means keep this feast that cometh in Jerusalem’. The best texts omit.” (Word Studies in the New Testament, by M. R. Vincent)
“The oldest authorities and the best modern editors, followed by the Revised Version, omit a large portion of the verse, reading thus: “but taking his leave of them, and saying, I will return again unto you, if God will, he set sail from Ephesus.” The words thus omitted are deemed to have been an insertion suggested by Act 20:16. It is not only on the authority of a small number of uncials that the words are rejected; their omission is supported by several cursives, as well as by the Vulgate and some other versions.” (Cambridge Bible Notes)
“The whole of this clause, ‘I must by all means keep this feast that cometh in Jerusalem’, is wanting in A[lexandrinus,] B [Vatican ms 1209,] E [Sinaiticus?], six others; with the Coptic, Ethiopic, Armenian, and Vulgate. Griesbach leaves it in the text, with the mark of doubtfulness; and Professor White, in his Crisews, says, ‘probabiliter delenda’. Without this clause the verse will read thus: ‘But he bade them farewell, saying, ‘I will return again unto you, if God will’.’” (Commentary on the Bible, by Adam Clarke)
Now, I quote some passages by a brief study (you may find the full extent of it in http://www.egw.org/zboard/325958) titled Acts 18:21 and the additions by scribes in the Byzantine times, by Koot van Wyk (DLitt et Phil; ThD) Visiting Professor, Department of Liberal Education, Kyungpook National University, Sangju Campus, South Korea, Conjoint lecturer of Avondale College, Australia.
So now we come to a case of a long reading for Acts 18:21 and a short reading for Acts 18:21. In the long reading, followed by the Old Afrikaans translation of 1933 by J. D. du Toit, E. E. van Rooyen, J. D. Kestell, H. C. M. Fourie, and BB Keet, Paul says the addition: ‘and said: I must surely (sekerlik) celebrate (vier) the coming festival in Jerusalem’. This translation may have led a number of individuals to insist that festivals should be introduced in Christian worship since if Paul is eager to celebrate the festival then we need to do the same. The conclusion from what one reads here is logical and probable based on this longer reading of Acts 18:21 provided that the manuscripts selected to do the longer reading from is stable, consistent, relatively older [although not a rule], void of additions and omissions in general, void of theological ax-grinding translation practices.”
[The short variant:]
“SBL Greek (Papyrus Bodmer [p74]; א (Sinaiticus); A (Alexandrinus); B (Vaticanus); Old Latin, Latin Vulgate, Coptic translations, Armenian translations and many other manuscripts.
ἀλλὰ ἀποταξάμενος καὶ ἀλλὰ ἀποταξάμενος καὶ εἰπών ⸃· Πάλιν ἀνακάμψω πρὸς ὑμᾶς τοῦ θεοῦ θέλοντος ἀνήχθη ἀπὸ τῆς Ἐφέσου”
[The long variant:]
“Codex Bezae Cantabrigiensis (Uncial D) and many late minuscles and Byzantine readings
ἀλλὰ ἀποταξάμενος καὶ ἀλλὰ ἀποταξάμενος καὶ εἰπών dei me pantwς thn eorthn thn ercomenhn poihsai eiς Ierosoluma· Πάλιν ἀνακάμψω πρὸς ὑμᾶς τοῦ θεοῦ θέλοντος ἀνήχθη ἀπὸ τῆς Ἐφέσου”
The short reading is supported by most of the earlier and important Uncials and Papyri Bodmer that is used for the reading. The Uncials dates from the time of shortly after Constantine like Alexandrinus, Vaticanus, Sinaiticus. The Vulgate of Jerome and the Coptic also supports the short reading. The Armenian also reads the short form. This means that they do not have the ‘festival celebration’ concept of Paul in it.
The long reading is supported by Uncial D which is called Codex Bezae Cantrabrigensis. This late manuscript has many problems and the Adventist scholars George E. Rice and Sakae Kubo, among others, did a great deal on identifying the problems of Uncial D (G. E. Rice, The Anti-Judaic Bias of the Western Text in the Gospel of Luke, AUSS vol. 18 no. 1 (Spring 1980): 51-57; ibid., The Alterations of Luke’s Tradition by the Textual Variants in Codex Bezae [Ph.D. Dissertation, Case Western Reserve University, 1974], pp. 174-222; also Eldon J. Epp, The Theological Tendency of Codex Bezae Cantabrigiensis in Acts [Cambridge, England: 1966] (non-SDA); also S. Kubo, The Nature and Quality of the Text of the New English Bible, AUSS vol. 5 no. 2 [July 1967]: 131-157).”
[The Conclusion of this brief study:]
“There is stronger evidence in favor of the omission of this ‘festival celebration’ passage of Paul than the inclusion of it. Building a case for a doctrine of reviving the festivals in Christianity cannot be based on this text, is a definitely a problematic cause.”
In addition to the fact that the ‘longer variant’ hasn’t the witnessing of the most important and ancient manuscripts (belonging to the correct textual criticism practice) professor Koot van Wyk points also to some interesting data about the Bible context (second method) of this passage of Acts.
I plainly agree with professor Koot van Wyk, adding to his arguments the witnessing of the Pauline talk itself, that we find in his letter to the Galatians (especially starting from 3:23).
In Col 4:10 Paul feared that his efforts to teach Galatians the New Covenant regulations were wasted. Why? He wrote: “You are observing religious days and months and seasons and years.” [NET Bible].
NET Bible’s note on this verse affirms: “The adjective ‘religious’ has been supplied in the translation to make clear that the problem concerns observing certain days, etc. in a religious sense (cf. NIV, NRSV ‘special days’). In light of the polemic in this letter against the Judaizers (those who tried to force observance of the Mosaic law on Gentile converts to Christianity), this may well be a reference to the observance of Jewish Sabbaths, feasts, and other religious days.”
In view of this, it can be possible Paul criticizes Galatian disciples on account of their observation of “Jewish Sabbaths, feasts, and other religious days” and – simultaneously – he told his brothers (according to the longer variant of Acts 18:21) he has an urgent need to participate to a Jewish religious feast in Jerusalem?
I hope this information will be useful to solve the dilemma.