Source: French, 32 years of living in France.
First of all, while we French have a reputation for going on strikes often, it is actually quite disproportionate. For example, I have myself not participated to a single strike, ever, and few of my friends have. I have, however, experienced a few :)
The strikes are more frequent in certain domains of activities, where the unions are most active. They are most frequent among government employees (notably in the Education sector) and current and former national/regional companies (notably in the SNCF).
With that out of the way, you have to understand that the goal of strikes is to pressure higher-ups in order to weigh on decisions (past or coming), and the pressure is generally applied by creating discomfort among the users/clients.
The result is that strikes are generally not announced too long in advance so that said users/clients do not have the time to take appropriate counter-measures and do experience the discomfort (and thus complain). The present situation is actually somewhat of a counter-example, with its long-ranging announces.
However, while the intention of strike has to be announced in advance (in public services), no single individual has to declare its own intention until the last moment. Though generally people express their own intent informally (to colleagues), they are allowed to change their mind one way or another (unless requisitioned, in some specific sectors such as security or health).
This individual freedom makes it very difficult to work out the exact impact of a strike in advance. Companies and other media agencies attempt to estimate it by polling, and thus obtain an estimated ratio of people on strikes, but (1) this is an estimate and (2) it does not specify exactly who will be available where and when.
For the case of primary schools, where classes start around 9am, it is fairly easy to take a roll-call at 9am and see which teachers made it and which sent a letter/mail informing they are on strike. Though of course, by this time the children are there already and those at work have to make do.
For other cases, though, where the individual schedules of employees do not line up on such a neat boundary, the impact of the strike is discovered throughout the day: each time an employee shows up, or let the employer knows she is on strike.
Thus, for Air Traffic Control or the SNCF, where employees show up a few at a time (not 9-to-5 jobs), the impact is assessed little by little, and flights/trains cancelled, delayed, rerouted, ... to the best of the ability of the management and present workers.
This means that even if you know there is a strike a given day, you can only predict it will be a difficult day for some people, but now who they are and how difficult it will be for them... if you need to travel on such a day, arm yourself with patience, and a few luck charms.
