What is the best way to organise seeds

When people say they organise by month.

Do you organise by the earliest possible sowing?

And then put back in for the next month for sucessional sowings?

Yes that is basically what I do otherwise I forget to plant things. 

I have my seeds sorted by family/type, but with some modifications.

For example all the peas and beans are together, but with root crops, those that need sowing several times over the season like carrots and beetroot are together and those needing a one off sowing date like swede are together.

There is also a section of seeds for quick salady things, herbs and leafy greens that can be sown right over the growing season, as these come out every time a spy a gap where I can get a catch crop in 

I use a notebook to list what can be sown month by month, but whether it does or not depends on the weather, space and how the previous crop is coming along.  If the March sown beetroot are slow to get going, the next sowing gets delayed so I don't needlessly make gluts.  The weather usually plays tricks and makes a few anyway, without any extra help from me 

All the bulky stuff like peas and beans go in large Tupperware tubs. I grow a lot of micro greens and they are just randomly together in another tub.
Everything else goes in a nice big monthly divider tin the wife got me. I put them in order of the earliest time I will be sowing them (not the earliest time the seed packet recommends, which is rarely accurate etc etc). For things I stagger sow, such as beetroot, carrots, lettuce etc, I then just stick them in next months slot and you can't forget!

Totty

I have opened parsnip and wonder should I just throw away.  Thanks

It is about the worst for keeping, so unless you have had ideal storage conditions, the answer to that is probably yes.  You could try chitting a few on damp kitchen roll to see if they are still alive