History of the Reformation in Regards to Holy Days.
During the dawning of the Reformation, the Roman Catholic church had established, and continued to establish, a plethora of feast days and holidays that burdened the believer with necessary observance by a necessity of faith. During these days, the believer would need to cease completely from their daily labors, do penance, and attending Mass; all in order to maintain their salvation.
In response to this twisted doctrine of salvation and assurance, many of the Reformers removed the church calendar and the practice thereof. But was this the final view of the reformers?
In the German city of Strasbourg, both Martin Bucer (1491–1551) and Wolfgang Capito (1478–1541) worked to reform the church. Initially, in Grund und ursach (Basis and Cause), Bucer rejected any day but the Lord’s day and removed the usage of the church calendar. However, later on, the Strasbourg Psalter of 1537 began introducing festal hymns to be sung during certain feast days. The rationale behind this change can be seen through a couple of his later writings. This reasoning will be flushed out further in this article.
Another prime example of a Reformed observance of feast days (From whom this article gets its title) is Francis Turretin (1623–1687). In Turretin’s Institutes of Elenctic Theology, while touching on the fourth commandment, he speaks about feast days. Turretin distinguishes between feast days as the orthodox (Those who are doctrinally sound) can observe them, and the way the Papists wrongly observe them.
The latter he unabashedly rejects because they made these days a necessity of faith, meaning it was necessary for the believer to observe these days, and to do the duties the priest instructed them to do. Turretin relates this to the way in which the Old Testament feast days were done. And thus, ultimately, rejects this way of observance because he rightly affirms that God alone has the right and authority to declare feast days as a necessity of faith. 
The former, however, Turretin leaves to the liberty of the Church, saying:
“ The question is not whether anniversary days may be selected on which either the nativity, or circumcision, or passion, or ascension of Christ, and similar mysteries of redemption, may be commemorated, or even on which the memory of some remarkable blessing may be celebrated. For this the orthodox think should be left to the liberty of the church. Hence some devote certain days to such festivity, not from necessity of faith, but from the counsel of prudence to excite more to piety and devotion.” — Francis Turretin 
The question for Turretin was not if these days should be celebrated (This, he says, the orthodox leave to the liberty of the Church), but if these days were more holy than others. Turretin denies that these days are holier than others, but says that all days (other than the Lord’s day) are equal. He then goes on to say that any holiness attributed to these days is not because of the time or place of their observance, but because of the Divine worship. 
Lastly, Turretin distinguishes four differences between the Papist observance and the Reformed observance.
“ If some Reformed churches still observe some festivals (as the conception, nativity, passion and ascension of Christ), they differ widely from the papists because:
1) They dedicate these days to God alone and not to creatures.
2) No sanctity is attached to them, nor power and efficacy believed to be in them (as if they are much more holy than the remaining days).
3) They do not bind believers to a scrupulous and too strict abstinence on them from all servile work (as if in that abstinence there was any moral good or any part of religion placed and on the other hand it would be a great offense to do any work on those days).
4) The church is not bound by any necessity to the unchangeable observance of those days, but as they were instituted by human authority, so by the same they can be abolished and changed, if utility and the necessity of the church should demand it.”