Arelith

I was looking over three of the holidays for Silvanus, 'Dryad Dance' on 30th of Flamerule (the 'Midsummer' eve) and both the 'Greengrass' and 'Highharvestide' holidays for the 'Song of the Trees' days (Some sources vary on this, but CandleKeep says both holidays are Song of the Trees for Silvanus' faithful).

So one bit of lore for Silvanus faithful is to break valuable items made of wood, and bury them, on these days as a devotion to the Forest Father.

While at first thought it be a thought to suggest adding burying things with the shovel item for this. I realized that there are a mass of other Faerun deity holy days devotions (http://www.candlekeep.com/library/artic ... ydays2.htm). So would it be possible, to on certain days, have an option appear at specific deities temples, to offer a sacrifice/devotion McGuffin item of sorts? Would it add to roleplay (would for me, but, well..)?

Holidays have been represented by Player Characters arranging In Game festivals, at their own expense, and incurring only the benefit of fleshing out the world of Arelith and making their religions feel more authentic. Given how easy it is to get piety through all means available already, turning the holidays of the many expansive religions of the Forgotten Realms into another shallow piety-grind would be a waste. This is not to discourage you from making the pedantic task of selling GP for piety more interesting; for example, you could sacrifice gold at an altar and emote that you are destroying wood and burying it as part of the holiday previously described, and even invite other players to join you in this ritual of appeasing S. Luditus Maximus on his high holy day of furniture sacrifice.

We watched those of Silvanus faith do this during a GreenGrass festival I held at the Myon Mithal a year or so ago, since as you said it is also a holy time for Silvanus too so they did their thing and people were respectful of it and then they came and ate lots of food!

Edit
To answer the question though, i dont think these things should become a mechanic, things need to be left up to the players of individual holidays to organise their faith accordingly imo

one easy way to represent this would be multipling piety gain by a small amount, and altar gain (through ceremony/donation) by a hefty amount, on the five IG days surrounding a deity's holiday.

this, of course, is a STAGGERING amount of work and likely not worth it.