For best results, order them 'in the green' from a reputable source in late winter/early spring. If you joined GoY, I could send you the details of a very good supplier.

And if you already have dry bulbs get them in the ground today.

I was weeding yesterday in the area where snowdrops grow and some came out with the weeds, sprouting already!

I found a couple of scilla shoots when I was clearing leaves this morning.

Modern thinking (via the RHS) is to plant fresh bulbs for best results. By fresh bulbs they mean ones dug out and planted without being dried off. Sadly many of the ones sold in packets in Garden Centres etc have been dried out and are basically dead. Some places do sell freshly lifted ones. Bridgemere Garden Centre used to have a good selection and if you went early enough, they were still in good condition.
You might be able to revitalise packet ones, by soaking overnight in tepid water with a little fungicide added, but the longer they have been out of the ground the less likely that is.

I've managed to get colonies of snowdrops started in two different gardens using dried bulbs Owdboggy. While not all of them will take, enough grow to make a good clump in a few years.

Cannot say that buying 100 bulbs so that one or two survive appeals to me. Rather buy 100 and have 98 grow.

Yes, in the green if possible or in August as soon as they appear in the shops/garden centres. Literally, plant them the same day!

Buying 50 cheaper than 5 appeals though, when half will grow.

The stockist I discovered, also via the RHS, sells them for £7 per 100 in the green. I do realise that these are G. nivalis, but I think that's great value for someone trying to get a patch established.