The Conciliar SSPX follows the Conciliar Church for the Feast of the Ascension

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The celebration of the Feast of the Ascension of Our Lord Jesus Christ on the fortieth day after Easter, always on a Thursday, is one of 'great antiquity,' dating back from Apostolic times: 

Feast of the Ascension

The fortieth day after Easter Sunday, commemorating the Ascension of Christ into heaven, according to Mark 16:19, Luke 24:51, and Acts 1:2.

In the Eastern Church this feast was known as analepsis, the taking up, and also as the episozomene, the salvation—denoting that by ascending into His glory, Christ completed the work of our redemption. The terms used in the West, ascensio and, occasionally, ascensa, signify that Christ was raised up by His own powers. Tradition designates Mount Olivet near Bethany as the place where Christ left the earth. The feast falls on Thursday. It is one of the Ecumenical feasts ranking with the feasts of the Passion, of Easter and of Pentecost among the most solemn in the calendar, has a vigil and, since the fifteenth century, an octave which is set apart for a novena of preparation for Pentecost, in accordance with the directions of Leo XIII.

History
The observance of this feast is of great antiquity. Although no documentary evidence of it exists prior to the beginning of the fifth century, St. Augustine says that it is of Apostolic origin, and he speaks of it in a way that shows it was the universal observance of the Church long before his time. Frequent mention of it is made in the writings of St. John Chrysostom, St. Gregory of Nyssa, and in the Constitution of the Apostles. The Pilgrimage of Sylvia (Peregrinatio Etheriae) speaks of the vigil of this feast and of the feast itself, as they were kept in the church built over the grotto in Bethlehem in which Christ was born (Duchesne, Christian Worship, 491-515). It may be that prior to the fifth century the fact narrated in the Gospels was commemorated in conjunction with the feast of Easter or Pentecost. ...

-1907 Catholic Encyclopedia

St. Pius X codified the eight Holy Days of Obligation for the Universal Church which, included the Feast of the Ascension, in the 1917 Code of Canon Law.

In the U.S. because of the influences of the different Catholic immigrants coming into America from all over Europe, the American bishops met in Baltimore to give uniformity to American Holy Days of Obligation:

When the Third Plenary Council met in Baltimore [1884], the bishops approved the uniform calendar of six holy days now observed: Mary, Mother of God; Ascension: Assumption of Mary; All Saints; Immaculate Conception; Christmas. The decision of the bishops was approved by the Holy See in 1885.

But as we all know, after Vatican II nothing went unscathed by the 'reforms' and in 1991, the US bishops removed the Feast of the Ascension as a Holy Day of Obligation.

It has been noted quietly for a few years that the now-Conciliar SSPX has followed suit in several of their U.S. chapels and now notes that the Feast of the Ascension is no longer a Holy Day of Obligation. Here is the bulletin for St. Mary's Kansas (for 2021) making it very clear the Feast of the Ascension of Our Lord Jesus Christ, a Feast honored since the time of the Apostles, is no longer obligatory in U.S. SSPX chapels [PDF here]:

The language from the Conciliar SSPX is now identical to that of the Conciliar Fraternity of St. Peter [see here]:

Sadly, this is yet another example of the slide of the now-Conciliar SSPX toward the modernism of the Conciliar Church, the Church of the Second Vatican Council, which we have been witnessing since the formal, never-retracted Doctrinal Declaration of Bishop Fellay on behalf of the SSPX in 2012.