As it says at midnight on the last day of the season so a transfer at 0001 is not part of the season. The rules regarding buying loan players says by the end of the season, which is midnight on the last day, or am I misinterpreting it.
How and when. The season ends in May the next transfer window for buying and selling players starts end of July which is next season. The football year finishes on 1st June.
They would have to buy them during the January window and then resell them back during the Pre season one as there isnt a transfer window facility to buy them at the end of the season.
Assuming that Watford do buy some of those players they will have to do so by 31st Jan when the window closes because they cannot buy anybody after then and still do it by the end of the season. Ist FEB could prove interesting if they haven't bought them by then.