1) Because every week, I get my pay check and I meet Nisab but the expenses throughout the week make it so, that I fall below Nisab.
2) Earlier in this year, I received my Income Tax check and definitely it met Nisab, however that money was placed on the side for about a month and then spent. Should I set that day as a Zakat 1st day?
بسم الله الرحمن الرحيم
The counting of zakah year shall start the day in which you receive wealth equal to nisab. If you own wealth equal to nisab on the day when you completed one lunar year, you will have to pay zakah. If the nisab fluctuates during the year, it may not have any effect.
Therefore, the day you become owner of nisab, if you have wealth equal to nisab exactly the same day after completing one lunar year, you will have to pay zakah. The decrease in nisab during the year due to expenditure may not have an effect.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.