My Schengen visa is valid from 5 July, but I need to get into France on the 28th of June.
Will I be able to?
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No, you will not be able to. You will almost certainly be denied boarding by your airline and, if you do somehow make it to the port of entry, you would be refused entry, detained and forced to return to your point of origin at the earliest opportunity.
And since you have a Schengen visa, it also means you actually require one to enter and therefore absolutely need to go through the whole application process.
For completeness, note that there are some exceptions, like seamen en route to their ship and members of an EU citizens family, who might be afforded the possibility to apply for a visa at the border. But that's not an option for regular travellers.