Small bulbs like snowdrops, crocuses, scillas and so on need to be planted as soon as possible or they'll dry out and fail but they can usually be inserted between plants that are still busy looking good. You can soak them for an hour in cool water to rehydrate them and give them a better chance of success.
Tulips can wait till late October or November as they can be prone to blight if planted too early. Daffs and hyacinths and alliums can wait a month as long ast they're kept somewhere cool and don't dry out.
When I was tidying up the garage at the end of Feb I found some packets of alliums I'd forgotten about and planted them out and, while not all of them flowered, many did and most produced foliage so I should get decent flowers this next spring.