Here's what the Wesminster Directory for Public Worship says about fasting:
Concerning Publick Solemn Fasting
WHEN some great and notable judgments are either inflicted upon a people, or apparently imminent, or by some extraordinary provocations notoriously deserved; as also when some special blessing is to be sought and obtained, publick solemn fasting (which is to continue the whole day) is a duty that God expecteth from that nation or people.
A religious fast requires total abstinence, not only from all food, (unless bodily weakness do manifestly disable from holding out till the fast be ended, in which case somewhat may be taken, yet very sparingly, to support nature, when ready to faint,) but also from all worldly labour, discourses, and thoughts, and from all bodily delights, and such like, (although at other times lawful,) rich apparel, ornaments, and such like, during the fast; and much more from whatever is in the nature or use scandalous and offensive, as gaudish attire, lascivious habits and gestures, and other vanities of either sex; which .i.we; recommend to all ministers, in their places, diligently and zealously to reprove, as at other times, so especially at a fast, without respect of persons, as there shall be occasion.
Before the publick meeting, each family and person apart are privately to use all religious care to prepare their hearts to such a solemn work, and to be early at the congregation.
So large a portion of the day as conveniently may be, is to be spent in publick reading and preaching of the word, with singing of psalms, fit to quicken affections suitable to such a duty: but especially in prayer, to this or the like effect:
"Giving glory to the great Majesty of God, the Creator, Preserver, and supreme Ruler of all the world, the better to affect us thereby with an holy reverence and awe of him; acknowledging his manifold, great, and tender mercies, especially to the church and nation, the more effectually to soften and abase our hearts before him; humbly confessing of sins of all sorts, with their several aggravations; justifying God's righteous judgments, as being far less than our sins do deserve; yet humbly and earnestly imploring his mercy and grace for ourselves, the church and nation, for our king, and all in authority, and for all others for whom we are bound to pray, (according as the present exigent requireth,) with more special importunity and enlargement than at other times; applying by faith the promises and goodness of God for pardon, help, and deliverance from the evils felt, feared, or deserved; and for obtaining the blessings which we need and expect; together with a giving up of ourselves wholly and for ever unto the Lord."
In all these, the ministers, who are the mouths of the people unto God, ought so to speak from their hearts, upon serious and thorough premeditation of them, that both themselves and their people may be much affected, and even melted thereby, especially with sorrow for their sins; that it may be indeed a day of deep humiliation and afflicting of the soul.
Special choice is to be made of such scriptures to be read, and of such tests for preaching, as may best work the hearts of the hearers to the special business of the day, and most dispose them to humiliation and repentance: insisting most on those particulars which each minister's observation and experience tells him are most conducing to the edification and reformation of that congregation to which he preacheth.
Before the close of the publick duties, the minister is, in his own and the people's name, to engage his and their hearts to be the Lord's, with professed purpose and resolution to reform whatever is amiss among them, and more particularly such sins as they have been more remarkably guilty of; and to draw near unto God, and to walk more closely and faithfully with him in new obedience, than ever before.
He is also to admonish the people, with all importunity, that the work of that day doth not end with the publick duties of it, but that they are so to improve the remainder of the day, and of their whole life, in reinforcing upon themselves and their families in private all those godly affections and resolutions which they professed in publick, as that they may be settled in their hearts for ever, and themselves may more sensibly find that God hath smelt a sweet savour in Christ from their performances, and is pacified towards them, by answers of grace, in pardoning of sin, in removing of judgments, in averting or preventing of plagues, and in conferring of blessings, suitable to the conditions and prayers of his people, by Jesus Christ.
Besides solemn and general fasts enjoined by authority, we judge that, at other times, congregations may keep days of fasting, as divine providence shall administer unto them special occasion; and also that families may do the same, so it be not on days wherein the congregation to which they do belong is to meet for fasting, or other publick duties of worship.
