In Letter 193, to his friend Arno, Alcuin writes:
Kalendis Novembris solemnitas omnium sanctorum. Ecce, venerande pater Arne, habes designatam solemnitatem omnium sanctorum, sicut diximus. Quam continue in mente retineas et semper anniversario tempore colere non desistas; adtendens illud et intente considerans, quoniam, si Helias, unus ex illis in vetere testamento, oratione sua, dum voluit, claudere caelum potuit praevaricatoribus et aperire conversis, quanto magis omnes sancti in novo testamento? ubi eis specialiter et patenter claves regni caelestis commissae sunt, et claudere caelum possunt incredulis et aperire credentibus, si intima dilectione honorificantur a fidelibus et coluntur glorificatione eis condigna. Quod ut fieri digne possit a nobis, lumen verum, quod inluminat omnem hominem, Christus Iesus inluminet corda nostra, et pax Dei, quae exsuperat omnem sensum, per intercessionem omnium sanctorum eius, custodiat ea usque in diem aeternitatis. Hanc solemnitatem sanctissimam tribus diebus ieiunando, orando, missas canendo, et elimosinas dando pro invicem sincera devotione precedamus.
On the kalends of November is the solemnity of all the saints. See, venerable father Arno, you have marked the solemnity of all the saints, just as we said. Keep that ever in mind and never cease to celebrate it on that annual date; attending to it and intently considering, seeing that, if Elijah, one of those in the Old Testament, with his prayer, as long as he wished, could shut the heavens to sinners, and open them to the converted, how much more can all the saints in the New Testament? for to them specially and clearly the keys of the heavenly kingdom have been entrusted, and they can shut the heavens to the unbelievers and open them to believers, if they are honoured with inward devotion by the faithful and are worshipped with the glorification appropriate to them.
And, so that we may become worthy, In order that this may be done appropriately by us, may the true light, that illuminates every man, Christ Jesus, illuminate our hearts, and may the peace of God, which exceeds every sense, through the intercession of all his saints, guard them until the last day. Let us precede this most holy solemnity by fasting for three days, praying, singing masses, and giving alms with sincere devotion for each other.
The letter is dated to 800, before March 19, in the MGH edition. An older text is in the Patrologia Latina 100, col. 296.
I’m not absolutely sure that I have the meaning of the last two sentences quite right, by the way. I am not clear what role in “Quod ut fieri digne possit a nobis” is played by Quod, except that clearly a comma is involved before ut; and indeed the clause seems rather odd to me.
Likewise “ea” later in the sentence must be the saints, and if so the prayer is for God’s saints who are still alive and on earth.
Update: many thanks indeed to Diego for correcting me! In fact “ea” is neuter so must refer to “our hearts”. The “Quod” references the last sentence and is the subject of “fieri possit”, and “fieri” needs to be “be done”, rather than “become” (both are possible). Also added omitted clause.