there is a big difference in the amount of tax you will pay since the finnish tax system is based on calendar years but the thresholds and exemptions are NOT pro-rated based on the month you arrive.
So if you move to finland in june your income from june-december is taxed as if that was your entire income for that year. if you move here in december then that one months salary counts as all the years salary - so you'll pay almost no tax on it. Move to finland in January and you'll pay normal tax from day one.
I moved from uk - there the tax year starts in april so it gets quite complicated as to when to move is the best from tax point of view for both the old and new countries.