They want to hold the election a WHOLE THREE DAYS before Johnson wanted to hold it.
UK elections are normally on Thursdays - which is Johnson's proposed date. I don't know whether the SNP and LibDems think they'll get some kind of electoral advantage by holding it three days earlier on a Monday* or whether they just don't want to agree with Boris - a sort of 'not invented here' attitude.
Either way, Labour are set to vote against - they really don't want an election until their opinion poll ratings are better - which may entail replacing Corbyn.
* students more likely to still be in their university towns in the run-up to Christmas perhaps?