On long haul it can be hard to avoide working weekends even with decent seniority. The length of trips is the factor:
5 days trips, only a trip starting on Monday has a clear weekend, the other 6 days impact the weekend, so only 1/7th of 5 day trips have a clear weekend.
For 4 day trips its 2/7ths that have a clear weekend.
For 3 day trips its 3/7ths..... but... if you did a Mon-Wed trip, its reasonably likely that you'll then be doing another trip starting on Saturday or Sunday thereby impacting that weekend.
Believe me, seniority does not mean you get all your weekends off! If someone says they don't work weekends, they must be top 2-3% of seniority, or part time, or going sick, or exagerating, or very very very lucky.