It is still unknown how the declaration will happen. The media supposes that the Catalonian parliament will gather for “analyzing the current political situation”.
This way, if Catalonia declares independence on the 23rd, it will beat the Spanish Senate which will consider the issue of suspending the autonomy of Catalonia on the 27th. This way, the vote of the Spanish Senate won’t matter, as Catalonia will be an “independent state” by then.
On October 1, 2017 in Catalonia, a referendum on the independence of the region was held. According to the results, 90.18% voted for independence with 43% turnout.
The Head of the European Parliament believes that the EU countries won’t recognize the referendum and Catalonia’s attempts to declare independence, so the region should abstain from trying.