A Monday is as good a day as any for planning decisions:
for the week ahead, or further into your future.
Ideally, whatever you plan, your goals should be SMART: specific, measurable, attainable, relevant and time-bound.
There are other suggestions as to what the five letters of SMART might represent:
S - sensible, significant, simple, strategic, stretching
M - maintainable, manageable, meaningful, motivating
A - acceptable, accountable, achievable, action-oriented, agreed, ambitious, assignable
R - realistic, reasonable, recorded, resourced, results-oriented,...