For the next 254 years it was actually compulsory in Britain to celebrate the plot's failure, with most choosing to burn an effigy of Fawkes on a bonfire.
Transcript and possibly helpful simplified version follow.
My lord, out of the love I beare to some of youere frends, I have a care of youre preservacion, therefore I would aduyse you as you tender your life to devise some excuse to shift youer attendance at this parliament, for God and man hath concurred to punishe the wickedness of this tyme, and thinke not slightly of this advertisement, but retire yourself into your country, where you may expect the event in safety, for though there be no apparance of anni stir, yet I saye they shall receive a terrible blowe this parliament and yet they shall not seie who hurts them this cowncel is not to be contemned because it may do yowe good and can do yowe no harme for the dangere is passed as soon as yowe have burnt the letter and i hope God will give yowe the grace to mak good use of it to whose holy proteccion i comend yowe.
My lord, out of the love I have for some of your friends, I want to make sure you are safe. Because of this I would advise you to not attend this sitting of parliament because God and man have agreed to punish the wickedness of this time. Do not think this is a joke, go to your estate in the country where you will be safe, because although there is no sign of any problem yet, this parliament will receive a terrible blow, but they will not see who it is that hurts them. This advice should not be ignored as it may do you some good, and it can do you no harm because the danger will have passed as soon as you have burned this letter. I hope God grants you the grace to make good use of it, and that he protects you.