I spoke to colleague yesterday and said i thought we’d agreed that we would talk about the best dates and perhaps take a week off each at Easter. She told me she didn’t think she needed to discuss this with me because ‘it’s standard stuff’.
That's a complete non-justification - just a slightly less harsh way of sticking two fingers up at you.
She was saying that she has plans for those two weeks at Easter
Her plans can be summed up as ‘to get the prime fortnight off instead of colleague’. Like most people – especially those with kids at school - don’t have plans for those two weeks.... 
People asking how she always manages to get in there first and why can’t OP beat her to it: some –CFs— people have no shame as to what methods they’ll employ. They’ll bombard/hijack the boss at a very inconsiderate time – leaving for a meeting, dashing to the loo, still taking their coat off having just arrived. Considerate people will avoid inconvenient moments, but selfish people will just barge straight in there. For all we know, depending on their working relationship dynamic, she might even be phoning the boss at home ‘for a friendly catch-up’ and then “Oh, by the way, I’ll need two weeks at Easter as I’ve got an important family event happening”. You also don’t know whether she’s a ‘requester’ or an ‘assumer’, who will mumble the vaguest of comments about her holiday plans and then later insist until she’s blue in the face that it’s all been discussed and agreed - or, at the very least, "You never said there might be a problem when I told you".
Just out of interest, is she an avid Daily Mail reader? Every single year, without fail, they have an article telling you how you can get ‘more’ annual leave, giving you all the dates to get off to maximise days around bank holidays. AFAIK, they never think to mention that you might want to discuss with colleagues to ensure that everybody gets a fair crack of the whip (or even whether your boss/job requirements will allow all of those dates). In fact, I think some people see it as an academic hobby to both maximise the number of full weeks off for minimal annual leave outlay and also to steal a march on their colleagues by jumping in there first.
If nobody tells you when the calendar is one how is she always getting in first?
Because she knows the right person to badger about it, rather than waiting for the announcement like everybody else. It might be her best friend who gives her the nod for this precise reason.
My manager is asking me if I have objections to all these holiday requests from her for all of the best dates. That's a very leading question. 'objections'? If you answer 'yes' you will sound obstreperous.
“Thank you for bringing this up: yes, unsurprisingly, I do have concerns about fairness in this matter.”
Moreover, I wonder if the colleague is getting in there first because she specifically has plans for those ‘best’ dates or whether she is just doing it because she doesn’t want OP to take them. I get the FCFS idea, but I still don’t think it’s necessarily the fairest way. It’s not a personal failing to not know from the beginning of the year every single day of annual leave you might want or need. Some people have highly routine lives, whereas others need more flexibility – childcare, health-related needs, family emergencies, job interviews etc. etc. – it doesn’t make one of them right and the other wrong. It’s interesting how people who have their whole month’s salary already ‘spent’ on pay day, with no allowance for whatever circumstances may arise, are considered careless and foolish; but those who prefer (or have no choice but) to see how things go and not rush right in are told it’s just tough: 'you snooze, you lose'.
It’s a bit like those people who were stockpiling toilet rolls and pasta in massive quantities – not because they expected to need all of it over the next weeks, but because they were afraid just in case others might leave them with reduced options. They had the time to go shopping, the money to pay for it all upfront, transport to get it home, big homes with plenty of storage space. Is it ‘fair’ for somebody who works long hours on NMW, has to get the bus and lives in a small flat to not be able to buy one pack of toilet rolls because somebody else has been in the position to buy 100 packs?