Here in the UK, the first day of term for teachers takes place a day earlier than that for students, and is spent in whole staff meetings, departmental or other smaller-group meetings, and some in-service training. If your remit is to ensure that ICT is taught either solely through other subjects, or by numerous people who teach just one lesson of ICT a week, getting colleagues to come to a meeting on that first day is virtually impossible.

And yet, if you're to ensure consistency of standards, and high ones at that, it is essential that they do attend. Or is it? Here are seven techniques that have been found effective.

Admit to yourself that it is unreasonable to expect someone who spends just 5% of their week teaching your subject to spend much more than that proportion of their first day preparing for it. By my reckoning that amounts to about 15 or 20 minutes. This "technique" won't help you get more people to your meeting -- but it may help you cope with the frustration of their not doing so.
Following on from point #1, think about whether you really need a meeting at all. Being realistic, even a whole day's meeting is not going to solve all the issues to do with consistency of teaching standards and assessment grades in ICT throughout the school. So a brief note in the pigeon-hole or in-box of everyone concerned may be a much better approach. And what should the brief note say? It should inform people where they can find the resources they need to do a good job.
There is no doubt that in many respects a meeting is better than just a note in someone's pigeon-hole, which they may or may not read. So, if you decide that you must have a meeting, be sensible and keep it short: no longer than 15 minutes.
Other subject leaders will no doubt be somewhat aggrieved if you arrange your meeting at a time that cuts across their own meeting. One way around this dilemma is to negotiate with the senior management a specified slot in the day. This can be arranged at the last minute if necessary. For example, perhaps the 15 minutes before lunch could be designated as "ICT teachers' meeting or familiarisation with the school's learning platform".
Do you have to have a meeting as such at all, as opposed to a series of one-to-ones? Given that you will want to spend much as possible of the day in your own area of the school, why not simply invite staff to drop in at some point so that you can give them a 5 minute briefing -- and that memo? If you want to avoid being continuously interrupted, you could ask them to come along within particular time slots, eg between 10 and 11, 2 and 3, and for an hour after school.

Whichever approach you take, make sure that you have a list of teachers you're expecting to see, so that you can cross them off as they arrive, and chase up the no-shows.
Another approach, which needs a bit of advanced preparation (but not much), is to mail-merge your note to staff so that each one is personalised with their name at the top, and ask staff to drop by your room at some point during the day so that they can pick it up. That gives you the chance to engage in a conversation with them if you both want to, and also indicates to you who has collected their "briefing".

In a way that is better for you as well, because it means you can spend more of the day on other things you may need to do in order to make sure everything is "good to go" when the kids return on the morrow.
One thing you may wish to do is to arrange a meeting with your colleagues some time after the first day, but within the first week. You could use that meeting to emphasise the really essential points they need to know and understand, and to check whether they are experiencing any difficulties.

One thing to bear in mind is that as professionals, your colleagues will want to do the best job they can, so their reluctance to give an hour of their first day to an ICT meeting may not reflect anything to do with that. Your responsibility is to ensure that they have the tools and guidance throughout the school year needed to do a good job. Their responsibility is to ensure that they do do a good job, and to come to you for guidance if they are having difficulties. So, although it would be nice to be able to have a "proper" meeting with your colleagues on the first day, it is not the end of the world if that cannot happen.